http://ecomodder.com/forum/showthrea...-em-10529.html
These keep engine heat in for the trip home, from the trip in.
They are the easiest, cheapest mod you could ever make.
The idea (since I don't run any bellypans) is to make the engine compartment like an open cardboard box, except upside down i.e. reduce the drafts that carry the heat away, and also reduce the radiated thermal losses. Of course a belly pan would make that even better, as would weather stripping and taping and really plugging up all the leaks, but I've seen good heat retention with the simple measures I've taken.
If needed they can be pulled out for summer duty... although, I don't pull them out any more. I guess I haven't worked my vehicles hard enough in the summer for it to become an issue.