I can't remember the thread, but someone was talking about trying to find some corrugated polystyrene like GoLite uses.
I found some at TAP Plastics. 2'x4' sheet for $5.50. The corrugations run in the 2' direction. They had some 4x8 sheets, but I don't remember the price.
I also found some tubes, epoxy and wound filament, light and stiff. Don't know how it compares to carbon fiber, but I picked up a couple of lengths of it. $3 for 2' or 3' length. They had varying diameters too.