After a ride in which I lost power to both my seat dropper and derailleur as I hadn't charged them, I went looking for a solution - to both reminding me to charge, and also reminding me not to forget batteries on my next ride.

I subsequently developed a remix of the SRAM eTap/AXS Cover for Battery Block from @antionoin_860226 but incorporating the keychain hole idea (e.g 'Remove Before Flight' tags) from @GlenF_15902 (thanks to both for the idea and work to date).

Example Remove Before Flight tags as follows: https://amzn.asia/d/0DjWZEy 

The hole is reinforced to prevent breakage of the piece.

My print took about 30min on Prusa Mk3S+ in PETG on 0.15mm QUALITY setting, with supports added only around the perimeter of the piece to ensure the overhang stays as modelled when printing (this is the part which interfaces with the bike).