Can you put a litter box outside? I have several in my outdoor enclosure, but I did have to build a shed for them. In your case, maybe a large plastic container would work Put a litter box inside and a hole in the side of the container for the cats to get in and out.

Since the old litter box area has been returned to it's former condition, maybe your cats just need to be re-introduced. When I move a litter box, I take the cat(s) that use it most and put them in it so they know where it is.