I've never seen a diagram ... but I did consider putting a rear bumper on our Roadster Pickup so I could install a bumper hitch.
I crawled under a lot of trucks to see what jury rigging was required and found that there are lots of ways to do it. Ultimately, I decided against it. "Homer" goes commando -- the way Henry intended.
I recommend you spend the summer looking at what other guys have done. Take pictures. Then, when you find the best, most solid way to do it, acquire the parts, install the bumpers, and publish a how-to in The Restorer or Model A Times.
If you make it look good, I might change my mind again.