I had the challenge to replace the battery of my wife's watch, which turned out to have a screwed backplate. Unfortunately, I didn't have the special tool needed to open this cover. After some failed attempts with two screwdrivers, I decided to try to print a tool. For sure I was sceptical about whether plastic could withstand the force needed to unscrew the cover but tried a print with PC Blend. And indeed this worked. Maybe PLA would have worked as well but I didn't try. I would not recommend PETG since this likely is too soft and would just wear off.

I also attached the OpenSCAD sources for people who have similar watches with different sizes of cover plates.

To unscrew the plate I found the best method seems to put the tool on the corner of a hard table and press the watch with your hand balm on top of the tool. You should balance your force to hold the watch firmly in place but don't break the glass cover. In that situation turn the tool on the table holding the watch firmly in place.