Today I needed access to my shower connections (I had a previous leak problem). I'm sorry that the picture I have is not showing the proper view. The top is the left side, the bottom is where you see the round shower screw on plate. Tilt your head to the left to see it correctly.
I cut a 10" x 10" hole with a box cutter. the plywood is very thin and it was easy to cut a perfect square. I am making a very thin picture frame around the cut out piece and will screw it back in the hole. It will look like a professional access panel when I'm done.
You can see that the plumbing fittings are the same as house fittings and they screw on from the bottom. your new shower faucet will need the same set up.
I HOPE THIS HELPS