We have a method of installing a 3cm corner shelf in an existing shower we call biscuiting.
1) Two slots are cut into each side of the corner shelf
2) Two slots are cut in the grout joint of the existing shower to line up with the two slots on the side of the shelf.
3) A biscuit (stainless steel washer or penny) is inserted halfway into each slot in wall.
4) The shelf is "dry-fit" onto the biscuits to make sure no adjustments are needed.
5) Epoxy is mixed and added to fill all slots and the shelf is placed tightly into corner so that excess epoxy will squeeze out between shelf and wall.
6) After epoxy dries the excess can be cleaned up with a razor blade.
7) The installation can be finished off with a color matched caulk.