It may have been suggested earlier, but have you considered scaling a real coupler in wood or other material to a size and weight that can be supported by a receiver hitch? A 6:1 reduction would give you a 6" long model, assuming the original is 36" long, with a shank that could be inserted into the hitch receiver. This also would give you the satisfaction of creating the gift from scratch which is your original aim.
That is what I'm going to do, but not 6:1. I'm thinking 3:1 or so, but same idea. With it being him, I certainly want it to be noticeable to other drivers.
Try contacting these folks. They are a major two foot gauge preservation and operation effort in the USA. They might know of a scale drawing for their smaller style of coupler: http://www.wwfry.org/
