As a tinkerer with classics (but also modern (er) cars) you have found that with the door panels (and even more interior panels), the fabric, the skai, the leather or plastic leather as the basis for the design from cardboard or in the most favorable a kind of hardboard exists. The upholstery is often glued or stapled. One leak in the vehicle, because a window has remained open during a heavy rain shower, the drain holes in the doors have become clogged and more scenarios can be imagined, and it happened with the beautiful 'tightly' covered panels. Moreover, the staples rust and the picture is complete. Now there are hobbyists who replace this base with MDF panels. Bit thick and therefore not beautiful at the end. (Water-resistant) plywood will eventually get started with your careful and well-intended craftwork. No, you can achieve better results with Masonite. For sale at any hardware store ... Carefully remove the cover from existing panels, place the cardboard on such a Masonite plate and mark it off. Then make all the cutouts and - if still usable - fit the cover. You can do that with the appropriate adhesives ... Remember that there is often also a thin layer underneath the door covering foam is applied. It's not for nothing! If 'expertly' carried out a repair for eternity.