Hi, Baj
I like the way you think!
I'm a make-it-work type myself, and am always looking for readily available materials to do a job they might not have originally been designed for...
I have to say that I'm interested in the spray-foam insulation suggested, but having worked with it before I find its "expansion" properties a bit intimidating in a situation like yours- I'm not sure how one would contain it while it dries.
I think either of your two alternatives would work well- any leftover materials could be used for insulation elsewhere in the trailer!
Don't forget about good ol' duct tape when you're putting your "blanket" on.
I
know you have some- you're a do-it-yourselfer, aren't you?
Francesca