A couple weeks ago, I ordered a gennou head from a purveyor in Tokyo and while it arrived last week, it took me a little while to study up on how to make the handle for it and I finally got to it today. The geometry that goes into the design of the handle was surprising with many factors being considered that I would never have thought of, including things like hand placement, air resistance, air turbulence, and even pinky placement. There was a lot to consider, as well as drawings to be made, and I admit that I got a little impatient and just started cutting. While the instruction stated that you would probably end up making three handles before arriving at "the right one", my deficiencies were highlighted in the final handle - I foolishly thought that I would make a clean handle to rival the illustrations. Ha, joke's on me.
Leading up to it, I had read a bit on proper preparation of the head - ensuring that the walls inside the eye were straight, parallel and the outer edges clean and sharp. The head that I was able to source was made by a master blacksmith whose ability to craft perfect interior walls and sharp edges solely during the forging of the head was astonishing, as I did not have to do any of the preparation that I had been studying up on.
The handle was made from Koa sourced from a ranch on the Big Island of Hawai'i.