+1 for the rivetted fish plates, although I'm not sure you even need to do that.
This thing doesn't need to hold water, just sand. Maybe a bead of caulk in the corners would do it? You could caulk both side if you want it "pretty". At 2mm thick, it's not like the edges are going to be flimsy and want to flap in the breeze. Probably the only time the edges would want to flex is when want to move it out your way for storage.