I simply installed an 85 dollar Home Depot window AC unit within my tongue box. I cut a water-proof hole from tongue box thru the front wall of the TD for the cool air to enter, with a scoop diverting the air upward and a return air opening below, aligned with AC supply and return vents in the AC unit. Just too keep things ventilated, I added a grille in the front of the box to discharge warm compressor air. I have had no problems with moisture or water from condinsation within the tongue box. This set up works so well, I believe it would freeze ice on the windows, if left on with the windows, doors and roof vents closed...... :-)

Roly, So Cal TD designer, builder and user!