Hello
I did a similar job in January. I bought some ply (probably less than 1 cm thick) in B & Q and cut to the correct size for a normal bath panel.
I had considered putting a door in the panel for maintenance access, but it proved to be too big a job, requiring a change to the frame etc. I had intended to use low profile hinges instead of magnets - good idea though.
When I tiled the panel, it turned out to be very heavy, although it looked great. To gain quick access to the plumbing, I attached picture wire to hooks at the back of the panel, and hid them at each side of the panel. In case of emergency, I just need to pull the wires at each side of the panel and it will come free. I initially thought that using a plunger would do the same job, but was afraid that it would pull some of the tiles off the panel.
SM