I use a inner tube for a tire I found at the local auto parts store. I got the smallest size I could and it works like a champ! I actually removed the pass thru window entirely and the inner tube made a perfect water tight seal against the elements. The kids were able to crawl through and use the restroom while traveling (which I don't recommend doing anymore). It doesn't matter how your camper lines up each time as it will conform pretty easily.
Looking past my kids hard at work at their legos, you can see how i removed the pass thru window and added the inner tube

This is proof it worked as we drove through a huge winter storm through Utah and Colorado and not a drop of water entered the camper
