A house is divided into apartments, one on each floor. For one floor which is sometimes unoccupied for lengthy periods, there is a water valve to shut the water off for the whole floor. This mitigates against water leaks developing when the floor is vacant....
Why not just add a hatch to the box you considerd so you can open the box and access the valve? It shouldn't be too hard to find a prebuilt valve box that looks decent in your bathroom. Most I know are for outdoor applications but no reason why they can't be used indoors. And building it yourself also shouldn't be too hard.
To make it look even better you could also consider putting a removable shelf on top of the valve box. That way it's more hidden and you have a spot to put soaps or other bathroom essentials.
That might be a good simple approach. I’ll look for outdoor plastic boxes that have a rubber seal. Maybe construction adhesive could attach tiles to it.
It's the best approach, also for cleaning. Extra stuff like that in the shower gets lots of soap residue stuck on it and can become moldy or attract bacteria. People need to be able to clean it easily and that handle is not easy to clean. Also cleaning agents may cause the parts to corrode. A little sealed box will solve all of that.