Getting the correct curve inside the frame is tricky and I made a bit of a mess of it on the first Plycycle project where I tried to drill out the hole then cut it in half. This time I took a more measured and staged approach. First I used a router to cut away the inside to a 'V' shape, then used a small one inch drum sander on my drill to get the curve. A rasp ironed out any lumps and bumps. It will need a bit of refining which I will do using a spare bit of head tube with sand paper wrapped around it which should create a tight fit.
Head tube first fit.
Head tube fit video.
However as I was working on the head tube I dropped it and put several large dents in the paint work which means I will have to rub it down and start again. It is not a big problem, more of a nuisance.