I’m building a small box in the shower which I will tile over. The box will cover a valve and must be semi-accessible. The valve will actually be controlled from the other side of the wall, so the box need not be opened routinely -- only if the fitting leaks or goes bad will access be needed.
I have an unlimited free supply of Ikea pressboard because neighbors throw it away daily. But I would not use it in a shower. Ikea pressboard is like a sponge with water. It’s also too thick. I have rigid space constraints. No limit on the box height, but the walls of the box must be thin (e.g. 10—15mm thick is what I have in mind). The construction board seems well suited for wet areas.
This is the old box, which was previously a solid block of concrete:
My building style is to make everything accessible and servicable. So in that pic you see a floor tile around the box. That tile is actually removable. I went to great lengths to give a solid mating subfloor but without gluing it down. So if needed I can just remove the caulking around that tile and get under the showerpan. This is why the box walls must be thin. If the box is big enough to overlap the floor tile, then it would ruin the accessibility of the floor tile. Unless I somehow figure out how to make the entire box easily removable.. then it wouldn’t matter if it rested on the floor tile.