Cargo lid

I have a ‘63 Valiant Station wagon. The hinged lid on the spare is plywood; I assumed it was a replacement for a rusted out panel. I asked John Fowlie (Big M Automotive) if he had a replacement panel. The two wagons in his yard have plywood panels as well!

Could plywood have been standard issue for the spare cover? Or is it common for these to rust out, and those owners did what mine did? If so, any leads on where I might find an original?

Dan