eBikes are getting more popular all the time, so it is easy to buy a complete ready to ride eBike. But it wasn’t always so. For years I have been making eBikes by converting regular bikes with a kit consisting of a motor, battery, controls and electronics. Ready made eBikes often have specific mounts for the battery, but for conversions you need to get creative. The most popular ways to mount a battery are on a rear rack, on the down tube using the water bottle “braze ons”, or strapped to the frame.
For my latest eBike conversion project I made a different kind of mount. I mounted a “triangle” battery in the front triangle of the bike frame with two custom “cradles”. This allows for a very large battery. The cradles are riveted together “L” section aluminum stock that support the battery on the bottom and sides. The cradles serve as both a mount and a partial case/protector. They are more compact than a bag or case and have proven to be durable. By using both water bottle mounts the system is stronger overall than a down tube only battery mount, and in particular it gives the battery much more side to side stability. This mount allows for a low center of gravity, near to the center of the bike lengthwise, and is supported by the biggest and strongest tubing on the bike.
This is not a full how-to plan with lists of parts, dimensions, etc. If you make one yourself it will be custom, so what matters is the overall design. Think of this as a baseline design with modifications as needed. I will share the modifications I had to make as examples. You may not need them all, and you may need to come up with some of your own.
In more detail: The cradles are made with two pieces of L section aluminum stock like you can get at most hardware stores. I used a medium thickness aluminum. The L stock pieces are connected with 2-4 cross strips of aluminum flat stock. Two of the cross strips need to line up with the water bottle braze ons and are drilled for the mount bolts, others can be at the ends of the L sections. The inside of the tray should be lined with padding everywhere the battery touches. The battery needs a strap routed outside of the cradles over the top of the battery since there is no top cover. Locating the strap is between the braze ons will prevent it from sliding past them. I used aluminum pop rivets to connect the various pieces of aluminum stock. Done right, pop rivets are light, low profile fasteners, plenty strong enough for this application.
I was hoping my choice of materials and construction would be durable, and based on a year of testing, it has been. I chose aluminum stock and rivets makes to it light and easier to work than steel hardware.
Note: these pictures may make the cradles more complicated than they really are. They are a simple design with a couple of twists thrown in, and a few mistakes visible in the pictures. If I could make them over again I could fix some of those mistakes, but they don’t affect the function of the cradles so I am leaving them as is.
The cradles have passed the test of a year of steady use. 
You can see how the strap is routed, where the cross members are located, and the rubber liner. 
Note the pop rivet locations. I used rubber on the bottom of the end cross pieces to protect the frame. One side is attached by bolts instead, for battery access. Only 3 of the bolts were used, though a 4 was prepped for.
The seat tube cradle installed, battery removed. 
The frame bolts need to be reached through the inner cradle. 
This shows the cradle in cradle design. The outer cradle aligns with the bike frame, the inner one supports the battery.
As for tools, a pop rivet tool is required. I had a little learning curve figuring out how to install a secure pop rivet so you might want to practice and find a how to video about that. I used a Dremel for cutting smaller pieces of aluminum stock and for trimming, buffing and rounding sharp corners. A compact circular saw is good to have for the bigger cuts. You could do big cuts with a Dremel, but it is slow and eats up a lot of cutting wheels. You will need a drill for the pop rivet pilot holes and frame mount bolt holes.
Here are the modifications to the baseline design I needed to make, your mileage may vary:
Removable L section on one cradle for battery installation/removal. The basic design is simple but you will probably need to address the issue of allowing the battery to installed and removed. If you have enough clearance between your battery and frame, you may be able to angle the battery and drop it into the cradle. But my battery fills up too much of the frame to allow for that. I made one L section of the down tube cradle removable. Instead of rivets, it is held in using short bolts, flat nuts, and slots that allow it to slide in and out.
This is the bolt and slot for the removable L section. The bolt is loosened, not removed, allowing the section to slide out and the battery can be removed. 
The flat nut has a low profile and is close enough to the side that it never spins out. 
Down tube cradle with the removable side removed. The 4th slot is not used. It is best to have the slots oriented this way.
Compensating for battery shape mismatch with the frame. The shape of the battery and your bike frame triangle will probably not be an exact match. To get the best support, the battery should rest squarely at the bottom of the both cradles for the entire length of the cradle. I made the seat tube cradle a “cradle within a cradle” design. The outer cradle aligns with the bike frame and the inner cradle aligns with the battery, and the two cradles are pop riveted together. This double cradle design solved another problem for me: my battery cables exit out the back of the battery in the center (side to side). That is where the seat tube is, and my battery cannot rest against that or it will eventually bend and break those cables. The double cradle also holds the battery away from the frame and gives the wires room to exit without a sharp bend.
Allow room for a front derailleur. My bike has a rear hub motor which allows for the use of a front derailleur and a wide range of gearing. I had to cut away part of the seat tube cradle to make room for my derailleur. There are many kinds of front derailleurs and mounting options, and many people don’t bother with them at all, so this is definitely a custom issue. I used a bit of trial and error to get mine to fit, marking it and trimming away bits of he mount until it worked.
It took several passes removing material to make room for a front derailleur. 
The lower cross piece has held up well , even with the single rivet and small contact with the L section on the right side. The double cradle adds strength as well as making a good battery contact surface.
The things I did to monitor the battery for the first year are good ideas to continue as regular maintenance. I periodically remove the battery to clean and inspect it and the mount. I have had to redo some of my rivets that were becoming a little loose. One of my frame braze ons fell off and into the frame. That was an issue with the quality of my bike frame, not the battery mount. I replaced it using a standard bike rivnut tool. The new rivnut is stronger than the original one and was pretty easy to install.
This mount allows me to install a big 1.6kWh battery pack, good for over 100 miles easily, maybe up to 150 miles under ideal conditions with minimal (but still useful) levels of assist. The end result is what I call an “adventure” eBike. Due to the size of the battery, I can use this for doing big all day rides like centuries, exploration, hauling loads, doing errands, touring, etc. without range anxiety. I am happy enough with the results to recommend it and share it with other eBike DIY builders.