This case + cap for TeleGPS is designed for being in a 38mm+ body tube with a 400mah LIPO battery.  It also allows for plugging into the USB port without having to take anything apart.

The cap must be used with the case.  The cap helps hold the switch and battery in place.  This allowed me to trim a little bit more off the size.  The cap is screwed into the case to make sure it doesn't move.  

All screw holes are built into the model and if should thread into the model without tapping.  Designed for 4-40 screws or metric equivalent.

I'm using a switch which you don't need to use.  That said, it is designed for this switch:  https://shop.gag.com/parts/push-switch.html

To stop the battery from moving up/down, I have one of the top screws for the PCB be longer in order to go all the way through.  This can be seen in the picture with the battery.