If I fly it before paint it would probably end up living its life looking like Checkoway's
Getting a bit off subject; I don't blame you.
As I live two blocks from the airport entrance, I had my RV completely bolted together about two years ago, unpainted.
Figured I could haul it over there late at night when traffic was nill. Then I found I had to remove the wings again to install nutplates for the wing/fusalage fairings.
At that point, I decided it was going to be completely painted before reassembly. I didn't feel like paying someone else 5- $8000+ for a paint job, 'cause I'm a tight-wad!
Painting is "major" if you're doing it yourself, and it seems like the plane is only 3/4's done again! It's hundreds of hours of work; especially when using four colors as I did.
Back to subject......
For the panel, I'd use the original as a cut out backing plate, and then overlay with at least three removable sections. This will allow some major work without being upside down.
However, I'm just a few short years from 60, and with the seats removed, and a few pillows plus reading glasses for closeup work, it's quit comfy under there, and you can easily go to sleep!
Getting out sometimes, sucks though!
L.Adamson