A little different than your question but I've been doing some door panel repair recently on my 240 due to my speakers pulling loose from their original riveted connections. The door panels are also thin fiberboard type stuff that have extra trouble around aging penetrations/connection points. The guy at my local interior car trim place told me to try fiberglass a try for reinforcing as that is a trick custom stereo install guys use. I picked up small fiberglass kit at my local Lowe's to give it a try. Worked great! You can dab it on / brush it around wherever needed and put on thickly it hardens up pretty nicely and definitely reinforced where I needed it. If you give it a try though make sure you have good ventilation as it is some very smelly stuff!