An idea of the lid that is used to close the filling sand hole. The red part are composite material and the white part can be made out of two layers of plywood (needed to be simplified).

Plywood and its cover (same material with the rest of the design) can be glued or bolted together. After close the lid, turn it 90 degrees to lock. The bottom surface (the square surface) is the surface of the design.

If you have any idea for the lid, please post it. We really need it!