Hello Tiimy,Thats a good suggestion about waterproof board.I used similar I think it was 3/16 masonite type material from the buiding supply.You just need about 1/4 inch foam as a backer.Don't go to thick or the handles wont clear the door panels and will rub.I also used a sheet of plastic to cover the inner door sheet metal.Keeps the moisture away from the trim panel.Ford used this from the factory and surprisingly they did not cover the openings in the inner door with small panels as they did prior to 1955.My 53 Ford had covers on the openings I remember.I used the 3M spray trim adhesive to attach the foam and also around the edges on the back of the panel to hold the cover material in place as well as 3/16 or 1/4 staples.These Fords are fairly simple to recover panels as they are straight not curved vlike some of the later cars.Also if you want to buy pre-made backing panels instead of making them I got a price from a trim company in the US who also do complete interiors but will sell just the bare panels and they are die-cut.If I can find the name i will let you know.ABC don't sell anything less than complete interiors. 

  

Check out your local hardware store .You should be able to buy a thin waterproof board .If you don't want to spend a fortune or are not worried about the correct interior ,why not make your own ??? Marine grade vinyl comes in many colors . Add an external chrome strip to the inside to make it custom( add drivers side exterior trim to  passenger door  inside )  . I've  seen some  really trick interiors done at home .Only costs a little time and imagination .Good luck.

Lon

yblocksdownunder

ABC interior is very expensive and i was wondering how other people repair there interior. Mine is ripped around the dor handles and I have to make a backing board for it