I just remembered an old Machinists trick/jig for tapers that will work for turning chisel handle tapers for socket chisels.
The Jig for finding the taper. A dowel with a hole drilled thru the center and then a slot cut in it, a smaller dowel to slide thru it, and a hose clamp to secure it in place.
The Jig for duplicating the taper.
Setting the taper.
After turning to approx size, use the Jig to TRY the taper carefully so as not to disturb the setting.