Here is the Simple, the most basic 4x8ft square to go on a Harbor Freight utility trailer. It has two alternative profiles depending on whether you want just a bit of 'teardrop-i-ness' or none at all:
Here are the plans: The Simple. I've tried to eliminate as much cost/material as possible - the hatch props are pieces of wood, but work semi-automatically, and the windows are just polycarb sheet with no frames.
Andrew