How do you make an access door for plumbing?

I'm getting ready to tile the kneewall in the pics below. I'd like to have an access door on the back of the knee wall that will allow for future plumbing repairs. I plan to tile the kneewall in the same two inch tiles that are on the walls. Does anyone have any suggestions as to how to provide access? I'd like to have easy access on the back of the kneewall from the floor to near the top of the kneewall. I've considered making a hinged door over this area and covering it with tile. I've also considered cutting the CBU along the studs on both sides and attaching it with a few dabs of caulk and then caulking instead of grouting around the panel. If we needed access, one could take a utility knife and cut around the perimeter of the panel and pull the tiled CBU panel off. Does that sound like a good idea? Any advice and suggestions would be appreciated.

Why would you anticipate plumbing repairs and why is that valve buried inside the wall?

Did you know that probably 99% of all plumbing installations don't have access for future repairs?

Your caulk idea is about your best bet if you want inconspicuous. Otherwise a hinged door would work.

I'm trying to plan ahead for the possibility of a future repair. It probably won't be necessary, but you never know. The valve is a shut-off valve and they are installed on both hot and cold water lines, but the cold water shut-off valve isn't visible in the pic. The plumber insisted on these and it sounded like a good idea. However, I would have preferred them in top part of the kneewall closer to the faucet valve.

Shut off valves should be in the basement.

Schluter has a kit called Magna Panel I believe. There are 4 very strong magnets that affix to the backerboard and the framing. They work very, very good. It the only product they have that I like, Sorry I just thought I'd throw that in.

How are you going to finish off the wall where the side of tub meets that vertical 2 x4? It you use 1/4 inch board then tile over it, it might stick out 1/2 of an inch. Just wanted to bring it to your attention.