Thin tin sheetmetal plate works real great and it's very lightweight. We have made our panels out of that and attach stickers to them for team number, school name, and sponsors.
Example:
The Orange panel:
http://www.chiefdelphi.com/media/photos/13930
Lexan works sometimes as well, but breaks a lot easier especially when covering a large open area:
http://www.chiefdelphi.com/media/photos/28951
Fancy Aluminum covers work well sometimes if you have enough weight:
http://www.chiefdelphi.com/media/photos/20375