I'm not "in love" with the look of aluminum sheets and trim for the sides and roof. The limitations of readily available sizes of aluminum sheets constrain my design to no more than 5 ft. wide, plus a small fudge factor Steve T. the builder of FishInn shared with creative use of trim.
I looked up alum. sheet prices and posted them in a thread in the Trailer and Chassis Secrets forum:
I made a trial alum. sheet order from SAF.com:
1 sheet of 0.063 x 60" x 144" ($2.78/sq. ft.) $166.80 (front and roof).
1 sheet of 0.040 x 60" x 120" ($1.76/sq. ft.) $88.00 (hatch).
2 sheets of 0.040 x 48" x 120" ($1.76/sq. ft.) $140.80 (sides).
The cost from SAF.com would be $395.60 to skin a 5 ft. x 10 ft. TD with alum. sheet. I didn't figure trim pieces into the estimate. This is over a 100 lb. order, so shipping would be free.
I like Filon. It costs a budget busting $3 per sq. ft. with much more waste from cut offs than using alum. And I'd still have to install the same amount of of alum. trim.

I bought Steve Fredericks cd. It's full of good ideas. I don't want to build a "woodie" which is the direction his lessons demonstrate.
I'd be pleased with a TD built with plywood for the sides and roof that are fiberglassed and/or painted in epoxy paint.
It could eliminate most of the alum. trim pieces, too, which I prefer for a cleaner look and water tightness.
An advantage, I hope, to using fiberglass and/or epoxy paint is that it frees me to more easily build my TD wider than 5 ft. The cost *seems competitive* to using alum. sheets and trim for a 5 ft. x 10 ft. TD.
Resolving this will get me a few steps closer to giving the muffler shop a frame plan and the go ahead for sparks to fly.
Thanks in advance,
Steve B. aka Esteban