How is the floor built, is it joists with decking and insulation under the floor decking? No insulation? Is it composite (wood-foam sandwich)?
ISTR you had asked previously about foam sprayed on the undercarriage? That would probably be the most efficient way to get a good job done. If you have heat issues with the exhaust, I would assume some shields could be added to accommodate that. Lacking the time, expense, or trust for spraying, my second choice would be to install rigid foam on any areas that were away from the exhaust. If I felt it was necessary to make the foam to undercarriage joint sealed beyond gluing and fasteners (I don't), a guy could overlay it with aluminum sheet and seal the sheets.