[elhigh]

How much flat surface do you need? If it only needs to be flat but being level to the ground isn't an issue, then you can simply have it running at a constant angle from bow to stern, no problem.

If you want something close to level to the ground but also don't need a very large work surface, then you could achieve your goal with a somewhat faster angle near the bow, but it flattens as you approach the rear until the final 2-3 feet are more or less level.

Tapering the sides in toward the rear is a no-brainer in my book. And the closer the bow is to the tow vehicle, the better. Aerohead has posted a lot of good info on aeromodding trailers for maximum slipperiness.

Finally, if this is an option, why not build in either a removable work surface or bench, or include a large pullout that will both make unloading the trailer easier and also be already level (more or less) to the ground? That can be pricey and adds to the curb weight of the trailer, but I think you would get a lot of it back in convenience and the freedom to optimize your trailer's aerodynamics would more than compensate for the weight and encumbrance.