Followup question:

I have to cut through plaster and metal lath. I guess it's expanded perf metal, from the tiny bit I can see through the faucet openings.

I would cut a hole from stud to stud, but it's a lot harder to find studs in plaster walls, at least for me. I hope the shower is centered between studs.

The web has recommendations for a Rotozip and a utility knife for cutting plaster down to lath. I have both, never tried the zip on plaster though. Or a dremel and a lot of cutoff blades. Anybody have a good way to do this job?