Welcome to ecomodder.
I guess we need / want to see pics of it to see if there's potential for improvement.
You'll need more posts to post a link (anti-spammer thing) but if you have pics online somewhere, you could post the URL without the http: // and www. parts.
By your description, you seem to have taken a good start though.
Light & none too big .
Radiused top.
Ideally, all trailer corners (front and sides) should be radiused but that isn't always easy.
Plastic drainage pipe - either faired in or cut in half to make it flush with a side - can be used, and can be made to serve as additional storage.
For fuel economy, the gap between towing vehicle and trailer needs to be reduced or even filled in somehow to prevent crossflow through the gap - horizontally and vertically.
There are a few threads on using aerodynamic trailers to reduce overall vehicle drag, like Phil's Toyota pick up thread :
http://ecomodder.com/forum/showthrea...ta-t-7839.html
Or BamZipPow's trailer project :
http://ecomodder.com/forum/showthrea...ler-26997.html
You'll get lots of ideas going through these
Reducing the gap horizontally can be done by putting vertical plates on the hitch - one big plate on the hitch centreline acting as a barrier , or multiple smaller plates mounted on the front of the trailer that capture an air bubble that does more or less the same.