Here are the photos of the wheel locking mechanism on my chicken tractor, as promised. To make the locking plates, I just drilled a large hole in a scrap piece of 1/8" thick steel, then cut it in half right through the center of the hole to end up with two pieces each with a semicircle cutout that locks into the shank of the lag bolt perfectly. You could also notch it out with a grinder or dremel tool. The plate is just screwed to the back of the 2x4 wheel arm.