Landscape cloth to let some of the water out after a rain, or galvanized wire against lovely gophers come to mind.

How about a cover for the top? Shade cloth the width and length with an overhang, plant stakes sewn into two opposite sides. (or a wooden cover by DH, one or two with handles to expose 1/2 or expose all.

Those are my thought. ams

I made a deck over ours that was basically two doors. I made a seat in the middle that the kids can sit on when in the sand box. The doors rest on the seat. When they want to get to the sand we just open the doors. When the doors are closed then it is their little island that they can dance on.

I bought one of those cheap plastic blue pools from Walmart and drilled water holes so that not all the water would stay in and not all run out. It has worked like a charm. I didn't want they kids playing directly on top pf treated lumber. call me crazy.

I made a sandbox with a cover, one that didn't fit tightly. It was similar to amts's idea. The chipmunks loved it. It made a nice, safe home for them. They all moved in! We chased them out so many times. We finally realized that if we left it open, they didn't feel secure so they abandoned it.