While you're building the heat boxes, I'd suggest finding a way to seal the flap valve. Right now one of the issues I'm facing is that the heat valves, when all the way off, aren't quite sealing. The floor in the area gets hot and some of the metal gets hot enough that you can't touch it, all just from leaked heat. This is only on one side right now, the other side leaks slightly. It's very hard to get to that area once the plane is completed, so I'd suggest sealing them up well during the build.
Just wanted to throw that in while you were talking heat valves.