When I built my big one almost like that , I made a bunch of brackets about 6" long by 5" high, they mounted on each upright, about 1/2 way up, and had a length of rectangular tubing welded on so that it extended from front to back, I used them to hold 4 karts verticle , with bottom held in a length of 2 x 4, with holes drilled to hold rear bumper bolt heads, then top was held with bongee or a srrap. also made some plywood panels about 2' wide with 2 x 4 on each side, so that a kart could be carried on each shelf. we also used the kart shelves so that we could sleep in bags, when weather permitted.