Replacing the back deck on my 55 Fairlane.

[Daves55Sedan]

Many of the resto parts suppliers sell pre-cut heavy cardboard package tray, custom fit for your model. You can order them in original colors or dye the cardboard with the color of your choice. Mac's and Concours have offered them, but possibly others too.
I actually cut up a tall cardboard shipping carton and glued crushed vinyl matching my seats and door panel upholstery. Lay it on the structural support under the back window and go underneath to poke the two holes for the hold-down buttons.