Just found the board this morning and registered to do some research, along the way I found your build thread here. Amazing work, and am looking forward to seeing the finished product. I did have an idea for your head door hinges, but you've already moved beyond that point! I was thinking using a pin vice to drill a small hole in the top of the door and the door frame, as well as in the bottom of the door and the deck. A bit of wire glued into the holes you drill on the door would hold it in the door frame/opening and allow it to pivot as if it were actually hinged. Hope I am explaining it clearly enough!
I also saw your garage door and if you replace that refletivix foil with some foam board it will work better fighting the heat. It's about $15 for a 4'x8' sheet at one of the big box stores. I've been researching the same project for my garage since it does not have an insulated door and it can be unbearably hot in there! I left NY about 6 years ago, you'd think I would be used to the heat down here by now.