Our door panel backs are CNC-cut out of 3-layer plywood that's the same thickness as an original, unswollen panel (about 2.5mm). We use plywood because if it gets wet, the board will subsequently shrink back to its original thickness when it dries (the stock fiberboard does not).
These are very-accurate reproductions - every hole lines up perfectly! With your original vinyl and these boards, you can restore your own door panels. (Order the optional silver mylar strips for a cost-effective way to restore the 'silver' trim that's bonded to the original panels.) Instructions included.