My cats won't leave my bun alone!!

So I have 2 Xpens for my one rabbits enclosure. (8'x8') and I have been trying to keep my cats out because they have started using the buns litter box and just bother him and stressing him out. I have been clipping a sheet over the top but they are still getting in. I'm at a loss and do not know what to use to keep them out, help!!