I don’t know why they bother doing this over an oem door panel.
It would be 10X better to just make a mold, then a completely new interior door panel from that mold.
Time. That’s why. Custom car stereo is all about getting it in and out to start the next job. We were doing this 15+ years ago to make door and kick panels, using mdf as the speaker baffles, and a t shirt stapled all the way around the perimeter. Some poly ester resin on the t shirt to make it semi stiff, skim coat of Bondo, wrap it and go!!! LOL.
Yeah, it is cheap and semi easy to do, but man that thing would fall apart with time. It slays me because I see people on cable tv programs doing the same technique… mdf, fiberglass, resin, and bondo.
Problem with using all those materials is they are disimilar and will eventually rattle apart.