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If you are talking about the large rectangular holes in the door structure on the inside, the factory used a tar impregnated wafer board. Insert the board thru the slots on both sides of the opening and use three screws at the top to hold them in place. The factory used tacks there, but they fall out. I replaced mine with countersunk screws. For the waferboard, I substituted a piece cutout from roofing felt (cut to the same size and shape as the original waferboard.
BTW, all the '55/56 car models have this access hole and treated the same way.