I'm sure you've seen/tried this, but -- the underlay foam that has a mylar sheet bonded on one side?
I've successfully made a winter sitpad/tent floor R-booster out of this stuff. The mylar seems to improve the durability of the foam as well as the warmth. You've inspired me, Daryl, I'm now considering a vest, maybe with a velcro tab closure...hmm, pretty lightweight bit of just-in-case gear, for sure. I've even got some adhesive aluminum tape lying around to hold the thing together. Then I could leave the signalling mirror home, too, just wear it shiny side out and start dancing... :)