How to build a shower ledge

I want to build a shower ledge in a shower.
Like the one below, more or less.

The shower is already tiled, finished and in use but due to the rush to move in no provision was made for niche, shelf or ledge. I now want to retrofit somewhere to put the shampoo etc.

The ledge should have the same tiles as the shower on front, side and top. The top will have a 1 degree slope.

The ledge need be only about 900 wide, 800 high and maybe 80-100mm deep. Alternatively, I could make it the whole length of the shower (1500mm) if I can get more of the same tile although I think that is unlikely.

The catch is that I don’t want any wall penetrations as I’m paranoid about leakages through bitter experience. So I guess the ledge will have to be stuck on with adhesive. I’m somewhat tolerant about using adhesive alone here because, as it goes all the way down to the floor, it will bear its own load.

My current thinking is to build a box out of fibrecement, stick that on, seal around it, tile it, grout and seal again. That’s about as far as I can get with the idea. Anyone have any comments? Done something similar ? Can see a problem that I have overlooked?

Cheers
Arron

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