Roger, yes, "improved foam" is probably Cascade Designs' Holy grail. Something etherial in weight (and compressed space) but durable and resiliant.
But since about 50% of the Theremarest's area is open I figured that a reflective layer on the inside of the bottom skin sould help, if only by 5%, without much of a weight penalty, if any.
Hey, it's worth a try to see if it works. Reflecting heat BACK to the user is the object, that's why it needs to be on the inside of the bottom skin. That keeps down the convective heat loss in the "unfoamed" air columns.