My 71 Boler
door was original and had wood slats inside which had turned to a mushy pulp from years of water leakage in around the window. It was also very heavy from being water logged and a PO's weep hole at the bottom didn't do much good. I removed all the pulp and replaced it with 1" styrofoam insulation cut to fit (much lighter now) Another factory could have used something different inside or perhaps nothing at all, but with yours only a year older than mine, a PO could have rebuilt it and left an empty void. I also added a hand bend metal tube frame epoxied to the inside of the outer layer of fiberglass to help retain its curved shape. In hindsight, the only thing I can suggest that I would do different is to take more time to preserve the inner fiberglass skin by trying to separate the two layers as opposed to cutting it apart, I ended up remaking an inner skin using the outside as a mold covered in wax paper.