Once the firewall had the finish coat of protective spray, I brought it up to install it. It weighs a total of 12.2 lbs (if anyone's interested). Before installing the parts on the frame, I used 1/4" thick x 1/2" wide silicone teardrop self-stick weatherseal by Frost King in the gaps between the rivnuts. This was to help seal the gaps and to prevent vibration on the panels on the frame.

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I ended up using about 55-60 ribbed #10-24 rivnuts and 3/4"-long, stainless steel, button-head, 1/8" hex-drive bolts. Of course if I had planned this better, I would have aimed for a more package-convenient 50 total ... oh well, I know its secure now. Another thing I did for the bottom panel was to offset the top and bottom screws by about half a spacing. Without any real rationale, I just had the gut feeling that this would prevent and weird flapping modes or something in that bottom section that might happen with evenly-spaced rectangle gaps. Due to it being burned into my brain in engineering school (and it's also apparent in the truss structure of the chassis itself) I went with triangles.