A simple, low profile AAA battery holders.
I used battery clips that have a long edge that I use in my projects to bend them a bit so I can solder them to a PCB board. Specifically designed to have a very low profile to save space (battery will touch the bottom of the PCB).
You also have an easy way from the sides to pull the battery out for replacement.
The battery clips I used are standard ones. These are 9mm x 9mm without it's connection foot. A sample can be seen here: https://www.amazon.com/uxcell-Pairs-Silver-Battery-Spring/dp/B00GN3PN46
You can find the corresponding AA batteries size holder here: https://www.thingiverse.com/thing:3115171
I used PETG for this, just because it is more elastic and less brittle. Plus the fact that it is also more heat resistant (when soldering). However, This will work just fine with PLA.