Here's how to create an access hatch.
1) Cut a hole in the plywood skirt (make sure your framing and tiles are strategically located = takes some planning)
2) Install some backer boards.
3) Thinset the tiles to the plywood that you just cut out. For best results, use unmodified thinset with a liquid latex additive. Do not apply thinset where the tiles will overlap.
4) Grout the tiles on the plywood hatch.
5) Wedge the plywood hatch into place with some thin wood shims. No magnets needed.
6) Secure the hatch in place by caulking around the perimeter of the hatch with a colour matched caulking.
If done right, you cannot tell there is a access hatch. If you eve need access, simply cut the caulking with a knife and remove the hatch. See the attached before and after photos for an example.