Spray foam - most would never think of using this stuff, but it has a lot of really neat uses besides insulation.
Where I first found out about using it was from a fellow that hand fabricates auto parts and he used the spray foam to make parts from fiberglass.
I had cab holes in my 1979 Ford F-250 and it now is water and dust free!
I had a Buick Regal that rust had eliminated a lot of the metal Detroit had made the car from. Rocker panels were foamed in and fiberglassed - worked great! I also filled all the holes so that the car didn't leak dust in on a gravel road. The door though couldn't hold the spray foam, just too much metal gone and so in the winter with the wind hitting the driver's door - was a Cold ride! I found that spray foam Does have its' limits as to what it'll fill.

I also found out the hard way, once you start to use a can, you have to use it all. I thought I could re-use like a can of spray paint. Not so! let it set a while, no more foam! It seals itself shut.