I don't see what the big "no-no" is about using the foam and fiber, etc. There are products...industrial grade, even.... sold to make this type of repairs. Granted, the type of foam you are using isn't reallys suited for the repair, but when it does give out you can a better type.

http://www.fibreglast.com/showproduc...FQGPWAodt0RGRQ
http://www.shopmaninc.com/foam.html

Think about it. Cars with foam and fiberglass are being made out the yin-yang...and have been for a long time. They last long enough if you don't wreck them.

 

its not a bad thing to use fiberglass on fiberglass or foam on a boat but, not the best for metal. all it take is a little water to get to the foam to get the metal rusting again, also with a little heat the foam will could expand more and push the repair out.

 

You are mixing foam, bondo, metal, and paint - all of which will expand/contract at different rates and vibrate very differently as you drive down the road. It will eventually lead to cracking. Worse yet, the foam could trap moisture and only speed up the metal deterioration.

That said I understand your reasons for not welding in sheet metal. It's a budget thing. Just remember that what you're doing isn't a long term fix.