Thanks 200sols, that layout is very similar to mine in the sense our toilet is also next to the shower exactly like that except mines a rectangular tray.
I've already purchased the tray and from memory its 40mm but I completely get what you are saying.
We are forced to make a frame on the entire wall because the concealed shower unit is on the same wall as the toilet and that also needs a frame. I'm thinking since I'm losing some space I may as well get him to make a shelf within the frame, not sure what they're called but I think they look quite nice.
Think you are talking about a niche. One in the shower is nice for storing shampoo etc.