Check out this link: http://www.stits.com/pfdocuments/epoxy.html It's atwo part epoxy filler that's lighter than anything you've used and easy to sand.

Super-Lightweight Epoxy Filler
Only 3.68 pounds per gallon -- Bondo is about 10!
Epoxy Tough
Will Not Shrink or Crack!
SUPER-EASY Sanding
Great for Composites, Aluminum, Wood, and much more
Under $20 for a Quart Kit!

To order: http://www.stits.com/store/msc.html scroll to the bottom of the page.

It's easier than West Systems, lighter than West Systems and sands better.