Covering knob holes in bathroom tile
If you do choose to use tile, you could use silicone caulk as the adhesive. It will bond well and is waterproof, you will need to secure it with tape and perhaps lean a heavy object against it to hold it in place until it cures. Make sure the surfaces are clean and dry, and use apply a bead of silicone caulk around the entire perimeter of each piece to prevent water intrusion to the underside.