I have a ficus tree with a pot that is about 1 1/2 ft across that my cat's used to LOVE to dig in and use as a litterbox
. I got some plastic screen from a craft store (the kind that people used to use to make kleenex box covers and stuff like that? that you stitch yarn through the holes?) -the long ones- and trimmed it to fit around the base of the tree to the sides of the pot (2 halves). Then I got some fairly large(2-6 inches across), flat rocks and laid them on top of that (with space in between, so the water got in evenly
) and then, just because I collect them, put a couple of small frog garden figurines in it.
The screen keeps them from getting to the dirt to dig, it's tough and won't hurt their feet, and the rocks make it look nice and weigh down the screen so they can't pull it out. It's worked for 6 or 7 years. I hope this helps!