Here's another idea: Use a typical screen top, but pick up a piece of plexiglass that is cut large enough to fully cover the enclosure. This will accomplish a couple of things. It will support the weight of the cat, plus it help hold humidity in the enclosure and protect your BP from drafts.

To help train your cat to NOT jump on top of the enclosure, I have found that placing 2" wide packing tape, sticky side up, on top of the enclosure works wonders. After a time or two of jumping up there and getting tape stuck to his or her fur, the cat learns to associate being on top of the enclosure with something unpleasant, and will avoid jumping up there. It takes an occasional "reminder" treatment when the cat's memory starts to fail, but I've found this works well without risking any injury to the cat.