Ah!

Yes, I have just realsied that the door has a pronounced curve to follow the car's body. Hmmm, back to the drawing board...

Once glassed in, the whole structure should be pretty solid and stable (many kit car manufacturers actually glass-in a ply panel into their bodytub's floors - or am I thinking of bathtubs here...)

What I can do is to use two sheets of thinner ply, and laminate them together in the curved shape - they should stay like this when glue has dried.

What have other builders done here? Is the steel frame a 'standard' solution?

Cheers.