Dave, I don't know why those doors where made that way, as I said before, kinda crazy but looks like your addressing the issue. I still think once you view the Chavers videos it will give you a bunch of different techinques to approach your issue, knowing that the top of your panel can only be 1/4" thick doesn't mean that the thickness can't change lower on the panel to a thicker panel. I personally use 1/8' baltic birch for my panels, yes its not 100% waterproof but I used watershields on the door (plastic to cover the holes) also baltic is dead flat. For about 75 years automobiles used a warpable pressed board (cardboard) door panels with watershields and now everyone due to marketing thinks it has to be waterproof I believe (and these cars don't see the water of daily drivers) I am 100% comfortable with the product I used, I co-owned a cabinet shop for 21 years and I am very familiar with different products and substrates so the type is personal prefernence, my roadster used the same material and has been soaked! Your on the right track it looks like but I am interested to see what you do after seeing Chavers videos as I think you might be in way better shape than what some start with! The Hagan methods fine, and looks nice but was developed to sell product to DIY and pro trim shops I am sure aren't going that route, kind of like Rod Doors did in the past, I think you will save $$ and be way happier. Keep up the good work it will [pay off I am sure.