A couple of years ago I purchased a sun sheild from Walmart or Pep Boys.
 It is made of heavy bubble foil , silver on one side and colored red on
the other.  The thing is accordion folded.  A couple of elastic bands
keep it compacted when not in use.  I got the Van/Truck size.  I used it
for it intened sun shade in Florida. Works great.  Well after moved into
a colder climate, I was forced to lay on a cold gravel floor while
working on the van.  Out of desperation, I used the "sun shade"  as it's
about the size of a creeper, a little bigger.

I put the colored red side in contact with the gravel and layed on the
silver side.  I immediatly noticed how extremely comfortable this 1/4
thick bubble foil was.  Almost at the same time I could feel my body
heat being reflected back to me.  When I was done I put back beneath the
rear seat.  I only paid ten bucks for it.  Now seven years later I still
use the same one, a couple of bubbles are popped, but it still
satisfies, especially when the floor is cold, hot, or gravel.

Mark