sooo I didn't like the doubled capacity trailer I posted above previously. It was just too big, heavy, and the boxes sitting sideways made for HUGE wind drag.

So now I'm on my 3rd incarnation of the trailer, with a single, larger box mounted longitudinally. The box is a bit more curvy so hopefully that will help with wind drag. The whole setup is also about 10 pounds lighter. Whenever my son is definitely finished riding in the trailer, I will remove all the fabric, and will be able to support the box with just two aluminum angles instead of the big piece of plywood, which will lighten the load even more.

2nd version of trailer boxes are shown on the floor here: