-drill a hole to mount your dryer duct, RV sewer, or SCAT tube flanges in the two long sidewalls
-you want one hole near the top and one hole near the bottom.
-mount the 12 fan to the hole that will is closest to the mailbox door.
-MAKE SURE THE FAN IS BLOWING IN TOWARD THE MAILBOX
-test to see if you can open the mailbox door if it was glued to the back panel, if not mark and cut slots in the back panel with a hack saw or other thin blade to allow it to open fully.
-glue the base of the mailbox to the base panel where you traced it before
-glue the sides and top panel in place as shown.
(I think you can figure out how to do this without instructions. to be durable I would use marine epoxy and fiberglass tape, but this is also much more expensive) simple 1"x1" strapping glued to all the edges with construction adhesive or gorilla glue would also work but be slightly heavier.
-run a bead of RTV along the top of the mailbox and glue the front panel in place. you want this to be a close tight fit against the top of the mailbox curve so the airflow has to go over the sealed end of the mailbox as that is the warm end. (see second picture)