Many USA manufacturers will follow industry standard and purchase a bulk order from China to keep costs low. If you want something custom then you need be prepared to pay significantly more for a non-standard solution. Either you can assemble your own pack by purchasing and combining smaller cells to fit your desired space, then wiring them in parallel or you can hire someone to do this for you. I have a 3D printer and will be happy to work with you in creating a custom battery case per your specifications. I'll charge $40 to design and print the custom battery case and $20 for labor to assemble the custom pack, plus cost for parts/shipping... you have the choice to buy and assemble or get a turn key solution based on how much you are willing to spend.

Most people will buy a battery that fits their space, and then design a battery tray instead which will be far less expensive, I can work with you in creating a custom tray as well, same $40 to design/print the tray to your specs.

Another option is to buy a Touring Car battery strap system and use that method to mount your pack