Ours is a hit too......just wish we could stop the possums from getting into it and kicking sand everywhere!! (And leaving their dinner mess in it?!! I found a watermelon skin in there yesterday!)
We didnt have a clam shell but we had a sandpit and got a piece of shadecloth from Bunnings nailed one end to the side and nailed a piece of wood to the other side to make a cover to keep out the cat and leaves and stuff, worked really well. You should be able to do something similar.