Here is a photo of the cart I built that hooks up to my 3 pt,from 2 x 2 tubing using the back seat of a golf cart as the seat w/seat belts. I also installed four swivel wheels so that I can easily move it around in the shop, when not on the tractor.
Sorry for the two photo's. Didn't think the one had loaded & can't figure out how to delete the second one. Hope this helps! Good luck on building a seat.