Find an old wheel chair @ a flea market, one with solid tires, get some 3/4" angle iron, and then you can make one that will also colapse for easy transport and storage. Me and my Dad are building some @ the moment. the basic wheel chair frame colapses to lay flat and the wheels are large enough diameter to roll easy over any terain. The frames on the wheel chairs are STRONG but suprisingly light. it's all in the design. Yours looks a tad on the light side but looks may be decieving. On yours, I would build the rest with 1/2"-3/4" angle iron. Do it in a way that it helps the integrity of the rest of the frame. Some spots on your original frame may benefir from 1/4" angle iron laid along the tubing and welded. Also, when building you might take into consideration the average dressed weight of deer in your are. Deer carts in Florida probably wont have to carry near the weight of one in Alberta. If nothing else, you got wheels and bearings to use on a complete other frame if the metal on yours is too light. If it's thick enough wall to weld with an arc welder set @ 40-60 amps or a wire welder, it's thick enough for a deer cart! As long as the design distributes the weight and stress properly.