The last major step to ready the overhead for installation is to fabricate an air plenum. I don't want to simply pressurize the overhead or every switch and switch plate would need to be sealed or leak. So, in the interest of ventilation control, I need a plenum.
I first fashioned a foam core from pieces cut/sanded and glued with a dab of 5 min epoxy. I had a few gaps and discontinuities so I coated the plug with epoxy, sanded lightly and filled with Bondo. (note: Bondo is a polyester and will eat pink Home Depot foam if not coated with a layer of epoxy) I then covered the molding surfaces with a layer of packing tape as a release agent.
I propped the plug up on nails and covered the two functional sides with 2-3 layers of 8 oz. glass and epoxy. Extract the plug with a little fighting, trim to fit and using flox and glass strips fasten into the overhead.
Just a few more bits and I am ready to fasten the overhead to the canopy top. Wow, adding this overhead "modification" has resulted in a lot of work/time.