You can also use a two part epoxy filler called "Superfil". It's an epoxy base and doesn't shrink like Bondo or polyester leaving a tell tale ring. It's lighter than any filler you have used and sands well. It also has enough body to stand on its own.

Superfil is avialable at: http://www.stits.com/store/msc.html

It was designed for the full scale guys building EZs and other composite aircraft using 6 oz cloth needing a good light weight filler.