New version designed from scratch, thinner and stronger latch.
0.4 mm and 0.8 mm version, this is the battery tab push extrusions high.
Also, added form to make a silicon seal, 1.5 mm and 2 mm thick, goes between cover and batteries/transmitter.

You may need to print new cover for every replacement as the latch may break if you try to remove the cover.
The cover latch won't break if you insert flat 2mm screwdriver between the cover latch and the transmitter and slowly and gently turn to lift the latch, I test maybe 100 times w/o any damage to the latch.

I removed the STL for LR44/SR44 as I don't recommend it, use only the 389/390 batteries.

You need to grind the top of transmitter and make it 6.50 mm high, if possible make it flat, by doing that, make sure that you won't break the top battery tabs.
I put few pictures together to show the replacement and grinding off process hier
https://imgur.com/a/Dr62A22

I recommend the Energizer Silver 1.55V 0%Hg 390/389 MD batteries, using them right now and holding over 10 months with battery status in xDrip+ 315 (new was 319), they will last over 12 months, maybe 18.

User silicon (I used transparent Pattex Silicon) around the edges before closing it, let it dry for 24h.

Print with supports, see pictures for correct orientation.
It is best to use PETG as it is flexible, PLA will also work.