The corrugated plastic or they have a plastic that is smooth on the surface and looks "foamy" in the middle. Both are very light. Out here I get mine at a place called Tap Plastics. Lay the vinyl letters out on it and then cut to size. Drill it and bolt it onto the back of the rack or basket.
You folks realize that he's going to be riding around with this on his bike in the area of "The Motor City", don't you? Yup, Michigan, with No-Fault insurance.