I have a Northridge Tools belt grinder for Knife making....No lathe....I freehand an Octagon and then shape it with an 8 inch contact wheel.You are making some beautiful brushes. are you using a lathe then belt sander? After seeing your brushes I've been thinking about getting a blank and using the cheap harbor freight 1x30 belt sander to make a handle similar to yours simply because it's the only handle shape I see that I could use just the belt sander to make.
I cut the Stabilized block of wood in half and use G flex epoxy to bond it.Nice handle!
How many pieces make up the blank?