Assembly questions

I posted a thread earlier about making my own brush. I now have the handle issue sorted (a friend of mine who studies at the Fine Arts college here is going to make me a couple out of some nice kauri burl, rimu timber and/or jade), however I have a question or two about the assembly process.

Firstly, the rimu timber, although gorgeous, is fairly light. What do others do to increase the heft of the handle to put that weight at your fingertips where it should be? Would epoxying a small fishing weight or similar under the knot work?

Also, when I epoxy in the knot, do I need to fill the whole hole to the lip, or just enough so that the rubber/glue(?) base of the knot is covered? I'm looking at setting one of the knots rather deep as an experiment, but not sure how to glue it in properly.

Cheers,
Tom