a couple of Arizona teams use fiberglass, you can get it as "pultruded" structural shapes such as I beams, channel, angle, etc. The 1/4" thick version is about as heavy as 1/8" aluminum of the same size, it's very tough (bounces back instead of bending), can be worked with hand tools, can be bolted together, painted, etc. The I beam works great for mounting wheels and protecting them well, and the channel works great for crossmembers such as across the end. Cost is a bit high because you have to buy long lengths and get it shipped by truck freight, but you can use the leftovers for next year's robot.
Surprisingly not many teams have tried it....
Aluminum works well too, it's strong, light, not too difficult to work by hand, and available in many many shapes. Except a 4" x 4" x 1/8" I beam
