I have seen racks made out of plywood and metal bar stock, all types of metal scraps. Your imagination is the limit. On the other hand if you are doing it just to save a buck you probably won't. I have picked up bike racks out of the trash for free, gotten them from the LBS for $3-$5 off the clearance/used stuff table. Swapped odds and ends for them and of course paid full price for a set of Jandd Expedition Racks
Aaron