Umbra White Simple Ledge Wall Shelf

I have two small built-in toothbrush/soap holders over my bathroom sink, but no shelf. It's often inconvenient not to have a shelf above the sink, so I'm thinking of covering and concealing them under an 18-24" x 4" ledge shelf, and this would fit the bill. The shelf could rest level on top of the small fixtures I want to conceal, the it would have to be screwed to a tile wall. How is this ledge shelf meant to be installed--is the bottom open and the screw holes concealed in the back wall of the underside? Or do you screw it to the wall some other way? Thanks!