I can't speak to 3d printing them necessarily, but when I've cut slots I've always been told to use as close to the correct size file as possible. However, you need a little bit of wiggle room because strings aren't entirely uniform. So a 1.50mm slot for a .59 string would likely be just right with a 0.0014mm tolerance.

Assuming you get more than one shot at printing, I'd slot them one gauge size larger than what you plan on putting on it. For example I use Regular Slinky on mine so if I didn't have a set of files I'd print it for Power Slinky both for the sake of tolerance and in case I ever want to change string gauges.

Good luck, I hope this works out for you!