I don't know what kind of xpen you have but I think you can purchase tops for them. Or, if you happened to be looking for a portable xpen, this looks like a good option too.
http://www.amazon.com/dp/B00ME8EMP0?psc=1
I remember reading other posts with similar issues where their pups were able to climb out of their xpens. I think they ended up draping a sheet over the pen or buying a top.