Man, I'd shy away from metal filler. I use the stuff bedding rifle barrels to the rifle stock. It's mixed into epoxy just like microballoons is mixed in for modeling. And the mix is HEAVY. Metal gives epoxy an impact strength. And we really don't need that. Unless you normally crash so hard nothing remains, and you want SOMETHING to remain.

In practice, a microballoon mix can be quite strong. If you need it to be, do what combatpigg says and mix in some fiberglass flox along with the microballoons. It's really not hard to do once you've done it some. "Once you've done it some" is the hard part.