I found this fellow during a search on driving. I love to drive and was at his site. As you will see, he has a lot of very unique ideas, and I bet he has some on how to build a pony cart. http://www.bowersfarm.com
Just a thought.
My dad used to make them, but he never did make a blue print for them. He made them out of a heavy spoked bike wheel. The last ones he bought were right around $75 each. I have found numberous pony carts on craigs list and actions for far less than what he ever paid for just the wheels.