This frame is made from grey steel shelving. I cut the steel corner angle posts with a hacksaw and put the pieces together with nuts and bolts and washers. The shelving supports are L shaped so I just cut one side of the L and bent it to the rectangular shape. To attach the bike wheels I cut to one of the holes in the supports and just bolted the wheel with the original bike bolts. The wheels are garbage picks.
The curved handle is 3/4 inch conduit pipe. The single thickness turned out to be too springy so I doubled it up and this worked better. It is still a bit springy when I am carrying a big load. I guess I should add a third layer of conduit or use a thicker pipe. I attached an eye bolt at the front of the frame and at the back to have a spot for bungies to clip on.
The frame was easy to work with but quite heavy. Commercial trailers usually have aluminium frames.