I have used flour mixed with epoxy for filler. It lies somewhere between microballoons and cabosil as far as strength goes.

When I used it as filler I mixed in some cedar dust (that I collected while sanding the boat) to match color of the area I was filling. It worked great; especially since strength in a small filled in area isn't as important as a fillet might be. Plus, it is much cheaper than cabosil.

If I am working on an area that needs some strength, I mix in some cabosil and wood flour and wheat flour to try and match colors as much as possible.

Another benefit to wheat and wood flours instead of cabosil is the flours are much easier to sand than cabosil.