If you don't have the welding chops for steel, 80/20 aluminum is a great DIY option that simply bolts together. Bob Clagett from I Like To Make Stuff built an aluminum rack for his Land Cruiser.

2 x 3 or 2 x 4 stock lumber can be used for the base of the rack, and 5/8-inch dowels work great as clamp holders. A forstner bit with a drill press creates exact sized holes at the proper depth ...