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Keeping the width in the vehicles wake is the main goal, that's half the battle.
I think, if you ask around, that most of us find that it is height (of the trailer) that degrades FE faster than width.
While the total amount -- and shape -- of the frontal area of the trailer seems fixed, it is that an "aero" cargo trailer (trying to use conventional shapes) benefits more by having been increased in length (even though it extracts a penalty) over a heigtht increase (where total cubic capacity is unchanged).
Again, the shape of the trailer is important. Defining the job[s] defines the trailer. From there the ways to guide the air between tow vehicle and trailer become more limited. A trailer one must be able to stand up within is a far different cry than one which has a lid or other hard cover for cargo.
A trailer that is in service to such an extent that the two are rarely uncoupled for engine-on hours needs to be different than one that is used even twice per week. Or, month. The number of miles covered against solo miles is a percentage. And a small percentage is only worth so much work, IMO.
Second to that is the expected weight of the trailer. Manufacturer guidelines don't tell the story very well as hitch rigging has more to it than what is realized. On-road only, or some unpaved road miles is another. Etc.
Taking a stab at the thing with a common template (trailer frame dimensions, clearances, etc), is where I'd go next. And to start with a common-enough vehicle (for it need not be a truck as we know around here).
So, is the vehicle one which runs most miles solo, or under a load? Most of the aero decisions are right there.
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