Who would have thought, I figured that you would have radiant heat creep into the outer door skin. But thats really cool that you got the temp results you did. Before you posted I was thinking that it would be cool to have the door gasket in the 3/8 gap, but I am not sure how well it will work since it won't really have a surface to squish against. It seems that it will want to roll out of the grove if it snags while opening and closing.
I tell ya what, your door is by far one of the nicest looking doors I have seen in a while. The shape is right on, the fit is fantastic, and the vents just top it off. Not telling you what to do or anything, but I would think about putting the flange on the inside of the firebox, let the door gasket rest on it. And leave that awesome looking door for everyone to admire.
I see alot of cool stuff but your door design is going in the memory banks for my next build.
Thank You for sharing.