I made one, but didn't do plans for it. I did draw up some plans after for someone else, but have since lost the file. Brian's Model Cars has a good tuturial on how to go about it, making sure the ventilation will work and has somewhat of a plan on a booth.
http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=23&CurPage=1Good luck with it, building your own booth is a great sense of accomplishment!