W.E. Brownells used to sell a cutter insert for their handles called Concave in a few sizes. I have one ? Size and don't know if they still offer?
I use a carbide wheel in my electric hand-piece from defunct Carbide Checkering Tools to rough cut. Finish with Concave hand tool. Finish cut ends with small gouge, when two borders meet cut to making V groove with a ridge line down the middle.