I suspect ED in Maine above has the right idea. I read on this board a few years ago that the most efficient amount of stand off would be one inch of air flow between a heat shield and the item to be cooled (floor board). I think the man who wrote that was required to do an analysis for something on his job that involved layers of moving air for the most efficient cooling and one inch was his conclusion. I never read if he tried it on his car. It makes a lot of sense though. I plan to try it "someday".
Land Rover had the "stand off" idea figured out long ago for dealing with the heat problem. If you look closely at the roof of an older Land Rover Series 109 you will see a roof panel on top of the main vehicle roof panel that stands off with about an inch of air gap between the two. This helped to mitigate the searing heat of the African plains when exploring or out on safari by letting air circulate between the roof panels.