OH BOY!! Here we go and thanks for scratching open an old wound and pouring lemon juice in the sore. LOL!!!
Disclaimer: I'm NOT an engineer, metallurgist, construction foreman, or handyman. I'm just a person who wanted a heated blind.
First, consider the opening into the cab "SATANS BUTTHOLE!!!!!!!!!!!!!!!!!" Do NOT consider that a shooting portal, window aperture, or other hunting access. We tried to make that the entrance and main shooting portal by putting heavy duty hinges on one side and having the whole thing open. NOT a good plan. It worked for about 3 months and then the weight of the door itself warped the wood so the seal failed and the wind howled through that thing. Furthermore, the Plexiglas window we put in cracked when the door warped letting water in which rusted the piano hinge we used to open it. Snow and frost melted on the door and windows, ran down onto the floor, and made a better than average hockey rink for everyone to slip on, fall, and bust your butt, gun, bow, and your pride. Basically, the thing was beautiful when we finished it, but it went to shit once mother nature got a hold of it. Lesson Learned!! PLUG THAT HOLE with solid filler so it won't move, budge, or wiggle. If I were to do it again, I would plug that big opening with green treated plywood, seal the outsides with silicone and tar so nothing would get through there. Then I would apply an additional coat of water sealant to the outside and cut and frame the smallest opening I could get away with in that plywood and make that the BACK of the blind where you enter. I would then determine the size windows I wanted in the other 3 sides and cut suitable openings in the walls, frame them up for the windows, and seal them back up making sure there was over-sized drip molding to keep water from touching any part of the window, framework, or seal. I would cut an access panel in the side where I could keep the heater on the outside of the blind as there is surprisingly little room in those things for one hunter with a chair, backpack, and bow let alone two. Additionally, that "glow" from the heater would be eliminated or at least much reduced. My gut feeling is that I would put vertical openings into the sides as the super structure of the cab is about 18 to 24 inches apart if I remember right. Cutting one or more of those might make the thing a bit wobbly so to speak. Anyway, our attempt resulted in 4 days of work, one seasons sit, and the wind blowing the damn thing down the hill the next summer. Got 125 bucks for the aluminum at the scrap yard, bought some booze, and had forgotten the whole blasted event UNTIL THIS THREAD!! I'm going to mix myself a Capt. Coke, console my wounded pride, and continue to watch this thread in the hopes someone will do much better than I did. (Grin)