The last time I changed out a shower valve, I spent a lot of effort with towels trying to get the water draining out of the pipes above it from getting into the wall.
This time I planned ahead, and managed to design and print a quick gutter-type part that would go under the valve to catch any water that drained out of it when I started taking it apart. The back rim catches on the backside of the pressure balancing unit and keeps any water from getting back into the wall.
I printed it with black Prusa PLA because that's what was on my machine, and I turned the speed up to 300% because I wasn't worried about the quality. It still turned out just fine.
I discovered that 4" was a bit short, not because it didn't have enough length to get the water out of the wall, but because it was tough to balance it on its own because of the weight of the back rim. I could make it work, but I taped it to the wall a little just in case.
I added another file that I haven't printed with a thinner back rim (1mm instead of 2mm) and a 150mm long gutter instead of 100mm. That's what I would use if I were to do this again.
The author marked this model as their own original creation.