Thanks again for the feedback. Pictures would better inform this topic in terms of openings (the intake is 20" x 10"), clearances, air intake path, CCA location, CDX riser and such, but I think we're decided; I'll not attempt to insulate the box.
Chris, I think you pegged it. The installer put the furnace on a three inch open-ended CDX box with a short piece of ducting between the top of the box and the trailer heating duct. The furnace, with its decrepit foam seal, sits on top of this short duct. The other side of the duct was at one time joined to the trailer duct with metal tape, but the tape no longer sticks. The result was that a lot of hot air was leaking from the furnace output back into the enclosure space. Replacing the metal tape with some heavy duty aluminum foil and a little bit of high-temp RTV seemed to make a difference in enclosure temperature. It's still hot, but not as hot.
I'll put a spare Ignition Board in the cupboard.
Thanks again for your thoughts.
