My wife and I have been real happy with a 13' x 13' screen house/enclosure that we purchased from LL Bean for about 250.00. I know that this does not attach to the Airstream, but it sure makes those buggy days/nites plenty tolerable.
http://www.llbean.com/webapp/wcs/sto...=&sc5=Shelters
We bought one of the really cheap screen enclosures a Kmart. After going insane and spouting every imaginable profanity in all languages, we returned it. There are about 50 poles that have to be set-up just so, or you end up with a sad looking Hotel West Nile.
The LL Bean job is pricey, but can be set-up in a matter of minutes, with its' convenient shock corded polls.
Good luck.
Jonathan