some light reading for you

http://www.wind-tunnel-designs.com/deflectors.htm

I know that a friend of mine had a enclosed ~7x14 that we used to tow years ago all over the place, it was like pulling a parachute when there was a head wind or a 45 degree head wind ... he added one of the fiberglass/plastic??? bubbles (similar to the silver one shown here) and it broke the wind perfectly behind a pickup truck to the point you could barely tell it was back there and our mileage went way back up (several mpg).... I know if you buy a square nosed trailer, I would highly recommend buying (or making) something similar to help with towing.

here's another option Triton trailers that might get infomation to you via the net.