Structural Foam for chassis stiffening

I use it in large open areas in the body like between the rear quarters and wheel well area's.
It makes a good insulation/sound deadener, although I never considered it a stiffener.
I know it sure takes that "empty barrel" sound out of the doors and body, but you have to be a bit mindful of where it may hold puddles of water against the body metal.
If you pay attention to that you can use a stick or something and make a trough in it to allow water to be channeled away from places it likes to puddle to where it can drain out.

Dangit, I wasn't ever going to mention doing it because I didn't want to hear what the critics with no real world experience thought of it. :D