Picture a plywood box that is built around the drawbar that goes on your 3pth. Bring a bracket out the top for the top bar to attach to. Insert a 2"receiver into the rear of it for a trailer ball. Then fill it with concrete, leaving a hollow in the top to carry chain, etc. Build the box low enough so you can set it on the ground to unhitch it or when you want to use it as a brake when parked. Of course you don't need a real $30 drawbar but simply stubb ends with the pins for the hitch( I welded bolts on the ends of a piece of channel and drilled them for the hitch pins). I needed about 500lbs of concrete, forget how many cu ft it was.....