I don't have any cats, so I can't help you with that, but I too was concerned about Pixel jumping ON his crate and from there getting out of the pen. I'm sure he could have too because since then I have seen him jump a lot higher than that! What I did to alleviate this worry, was attach the ex-pen TO the crate. I have a big wire crate that I bolted the ex-pen to. It also gave him some additional room, as the ex-pen is an extension of the crate. I just took the door off, so it's always open. It's really worked out well for him and he loves having his very own "room."

Good luck!