Are you too cheap to buy a Fatshark battery and you just want to make your own battery pack (or did you empty your wallet with that shiny new Fatshark that you didn't have any money left for an extra Fatshark battery)? Here's a set of end caps for your DIY 2x18650 lithium ion cells so that you can attach the pack to the Fatshark strap.
The file "Fatshark 18650x2 DIY Battery Cap - End" is for the end of the pack without any wires coming out.
The other files have a slot either in the center, side, or somewhere in between which you can route the wires through.
You can print in any filament type, with supports.
You can also reduce the size to 89-91% so you can use 16650 lithium ion cells.
Attach with hot glue, double sided tape, or any non-conductive adhesive you have.
Any filament. Print with whatever infill settings you are comfortable with. Print right side up or upside down with supports. I used 30% with TPU with my prints. Play with the size percentage during slicing if your pack cannot fit into the end caps. Maybe our cells are of different size. Or maybe you wrapped the cells in electric tape more than I did.