My girlfriend's instax mini 70's back cover lock disappeared after a party. This is a should fit right in and is serviceable.

My printer MK3S+ settings:
Filament: Prusament PETG
Print settings: 0.15mm QUALITY @MK3
Supports: Organic Supports on build plate only
Infill: 100%

To install:

1. Use a thin device like a thumbtack to trigger the back cover lock mechanism to open where the original back cover lock used to be.
2. With the back cover open, find the metal tab on the inside rim that holds the back cover in place when it closes.
3. Using your finger, slide the metal tab. It should move the back cover lock mechanism as well. Slide the metal tab so the back cover lock mechanism hole (the one you used the thin device on) is dead center of the gap.
4. Snap the printed back cover lock into place, should be a snug fit.
5. Slide the lock up and down to make sure the metal tab also slides up and down.
6. Done.