A simple shower tray/shelf for adhesive mounting. Use construction type adhesive to glue the support rails to the corner. Make sure they are level, and place the edge of each rail at least 21mm from the opposite wall. The model has some leeway for imprecise mounting of the rails (or non-square corners), but not much. In case the corner is less than 90° you'll have to place the rails a bit further out. Stick the rails into the tray and check if your measurement is correct before applying the adhesive.

The interface between rails and tray should be friction fit to prevent it from falling off when you place something near the edge. If it is too loose, use your preferred method to increase friction between parts (paint, scoring, notching, heating and expanding etc…) You can also permanently attach it with super glue if you wish, but I prefer these trays to be removable for cleaning.

The hexagonal mesh is designed to effectively shed water by not providing sharp corners in which drops can stick. Also, the bottom of the mesh has sharp edges and points to make water droplets fall off.