Originally Posted by Jesse Open

Jim,
Lapping is not a good means to cut that seat adequately. A real, formed cutter is a better bet. For a one shot cutter on aluminum plain carbon steel would suffice. The angle of the cutter face should be very accurately cut, and a pilot bore provided on center and true. Piloting on the guide bore is your best bet. The actual cutting flutes could be milled, ground or even hand filed and finished by hand with a slip stone. You may want to visit a cutter sharpening shop and get some quotes or suggestions. Often the work can be had for a six pack of good brew. Consider as well a second cutter to at least level the top of the seat ring for a two angle job.