heh. I just have a LOT of years building custom crap, cars, dirt bikes, off road vehicles, home remodeling, scale modeling, sci fi prop replicas, all manner of one-off stuff. All about that improvise / scratch build. So I'm digging the whole 'build it from scratch' thing that you are working on.
I just finished hacking up a blown out exhaust manifold on my pickup to making a custom fitting adapter for my smog equipment on a close-match junkyard replacement manifold. The 'new' part had four small smog pipettes, the original had one large one, I'm on a very tight budget so I had to come up with something and no joy finding a 5/8" fine thread fitting that would match my smog plumbing. So I chopped the port out of the old one and reworked everything with a brass 3/8" NPT close nipple fitting to tie it all together. I MacGyvered the crap out of that thing, running fine now.
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Are you taking any steps to control or direct the sag of the panel when you glass it? Once it gets heavy with resin it's gonna sag down onto your bracing, no matter how tight you make the dry fabric.
I once saw a guy glassing a custom sailboat hatch cover that was a similar perimeter frame as yours, he put together a sort of jig for it using hoola hoops, suspending the framework between and inside the layout so he could glass it, slop the resin around and then roll it over so the gravity sag would be in the 'up' direction (away from the framing) when the panel was installed. Once that slight bow was set on the first layer he took it out and worked it on a regular workshop bench.
Or you could stand the door in a vertical jig while it is setting up.
