Instead of a wire x-pen - try a fabric one instead. The one I got from Mighty Mite has a zip up lid so even if they manage to tip it over (which is hard for them since the pen also has a floor and as long as they are standing on the floor, they can't really push the walls over) they can't get out. I have a jumper who was able to climb and jump out of pens. It was really annoying until I discovered the cloth variety of pens:
Folding Soft Sided Dog Exercise Pen