I am making a ledge approx 4in wide behind my bath taps so that the taps are not too close to the wall. I have made this out of 2 pieces of 3" x 2", one fixed to the wall and the second one fixed to the first. This is then covered with an offcut of 18mm exterior grade ply to bring it up the same level as the bath. I have cut the battens with an angle of 2 degrees to ensure that water does not lay on the ledge. I want to tile this but don't want to buy ply adhesive for such a small area. How should I fix them? Should I apply PVA to the ply first, and should I 'grout' these tiles with silicone?