We have used Alumalite with good success. It has a plastic core with aluminum panels on both sides and is used for high-quality signs and on the sides of RV's. It comes in various thicknesses (we used 1/4 in.) and is very rigid and strong. It also looks great, cuts with typical woodworking tools, is reasonably economical (4 by 8 sheet is about $80-100 from a local sign supply company). We use small bolts, nuts and washers to mount components. It comes in various colors and can often be used for other components on your robot. Check
http://www.laminatorsinc.com/sign-pa...ucts/alumalite for more information. Alumalite was used to make the trailers used on field a few years back.
We have also used corrugated polycarbonate sheets. These are extremely light weight, very tough, mostly rigid and easy to work with woodworking tools. This product is used in roofing and wall panels in greenhouses and can be purchased from your local plastics supplier. McMaster Carr also has a similar product. It comes in a variety of thicknesses. If you are mounting a lot of heavy components, I'd use 3/8" - 1/2" thickness, but light weight components can easily go on a 1/4" piece. To keep bolts from pulling out, use washers, maybe fender washers on heavy items. Again, this is not a very expensive product that will save a lot of weight. Do a search for "corrugated polycarbonate" for more information, or run over to your local plastic supplier to see it.
Dr. Bob