We used these when we had our PUP.
I know people have made their own out of solar blankets too but we really liked the super wind gizmos. We use them for 6 years without any issues and we also traveled 40000 miles and many many nights in the PUP. Bob was a great guy to deal with.
Worked great and kept out condensation on the bunks. Also we did keep out vent open a bit or a window corner zipped down just a bit to help with the circulation.
Hope this helps.