I own a Stuckx Bull, a weird chronograph from a Dutch microbrand that was on Kickstarter in 2015. My local Mister Minit didn't have a tool that could open it to replace the battery, so instead of taking it to a proper jeweller in the city like a normal person, I took it as a challenge to make my own compatible tool.

This is what I came up with - the inset shape is a dodecagon that measure 28.8mm from edge to edge in the model. I measured the caseback itself at 28.5mm, so 0.25-0.3mm extra clearance seems to be the sweet spot for doing this. This exact tool opens the back of my Stuckx Bull, and 28.5mm also seems to be the size of some Panerai models (and likely others out there as well).

I recommend printing this in PLA or ABS, something inexpensive and ordinary. The part you print will wear out quickly with just a few uses, so don't print this expecting it to last forever in your toolbox. I did try it with polycarbonate (also pictured), but not only does it seem to wear out just as fast, but I needed to print it with a 29mm gap (so +0.5mm vs the actual measured caseback width) for it to fit, so the clearance needed might be different with different materials.

If you've stumbled on this because you're looking for a tool to fit your watch, but this one isn't the right size, feel free to message me - this is very simple geometry to reproduce in different sizes, and I don't at all mind helping someone getting into a watch that otherwise might never be repaired.

Obvious disclaimer though, printing this and using it to tinker with a watch is done at your own risk - I'll be taking both this tool and my watch to a professional to do that part. This print just solves the first mechanical problem in that process and enables the rest if you have no other option.

Update 3/2/24: Updated the STL with fixed geometry, there shouldn't be any non-manifold edges this time.