A dummy 10440 cell (AAA battery).
You can use a paperclip to join the contacts, or a diode if you're trying to replace 1.5v batteries with 3.7v cells.
When finished, you can use a little dab of super-glue for the cover.
Here's the AA version:
https://www.thingiverse.com/thing:6644869
Mez_AAA.stl | 546.1KB | |
Mez_AAA_Cover.stl | 141.9KB |