The Aussies are the one major nation that actually use significantly more energy per person than Americans, so they're not the first choice for low drag examples!
For low drag fronts, this is pretty good:
Top and sides curve smoothly out into the flat sections.
Minimum-size fenders, and if possible tapering in and out rather than square/round, will help as they are right out in the airflow.
After that I would start to look at tapering the back in ('boat-tailing') in both height and width. Airflow will break away from anything more than about 15 degree slope, so it's gentle tapering we're after - all teardrops taper waaay too much to be low drag. Where the taper ends, cut off square.
So this one-off trailer a forum member posted a while back is pretty nearly ideal, except for the square fenders and the roof that slopes down too steeply - but then that horrendous air conditioner ruins any idea of smooth airflow over the back half of the roof. Of course this shape is a bit of a nightmare to build!
But the single most important idea for low drag is to make the trailer narrow and low. Making a low drag shape is hard for the amateur to do, but anyone can make a trailer small.
The limit is to make the trailer no bigger than the tow vehicle - I'm annoyed that I didn't keep a copy of a trailer made by Honda as an extension to an Insight coupe that actually had lower drag than the car on its own.
