Cut two pieces of 3/8 inch concrete reinforcement bar a little longer than their respective sides of the triangle mentioned in step 1. Weld them to the sides of one of the hex head bolts as shown. I welded on the wheel side of the bolt, too. Do this fitting and welding again for the other wheel truck. Remember that the wheel trucks will be mirror images of each other so the wheels are on the same axis when finished.
In the photo you see the wheel trucks assembled. When I did the welding, I simply set a hex bolt on a flat surface, head down. I laid the reinforcement bar next to the hex head and welded.