Put the front wheel on a swivel, like an over-sized swivel castor, or use the front fork/wheel from a 10-12 inch child's bike. That way, you could keep the 'T' shaped wheel alignment, have weight bearing support on the front, but have a rigid tongue, which will track much better than a wagon at 20 MPH. It's also easier to back up. I would go no wider than 3 feet outside dimensions on the rear wheels (30 inches would be better,) and use 20 to 26 inch bike wheels for the two rear wheels. In addition, have the floor of the trailer below the axles of the rear wheels - maybe 6 inches ground clearance or so. This will make the trailer much more stable.