I would try using 3M glue to laminate a SOL blanket to a fleece blanket and use that as an under quilt. The reflective surface should be on the bottom, facing the hammock. I would cut slits (or slots) in the reflective surface if moisture build-up was a problem.

Or, I would try a piece of the breathable reflective material I linked to in a previous post all by itself.

This stuff here