Skin ???

[G wizz]

a I'm building a small (3'x 5 1/5') cargo teardrop to pull behind a motorcycle, and will be using the small H.F trailer frame.
As I'm trying to keep the weight down, was wondering about the top and sides.
I have located enough aluminum travel trailer siding to cover the trailer.
Question is:
Will I need a plywood backing to glue this to, or could I frame up the sides and top with 1x2, pine, and attach the aluminum siding directly to that?
Also, where is the best (read cheapest) sorce for soft corner molding?

Thanks,
G wizz