I've seen several places that a large fly cutter works well on Sherline mills. Clickspring shows one. A large disk with stub cutter protruding. Part of the reasoning is a larger mass to hold a more constant speed. I have a Sherline fly cutter but it is a real light weight compared to Clickspring's.

Poking around I found an interesting concept. A flycutter build putting two cutters in a large disk. 180 deg separation between cutters but one further outboard than the other. The outboard cutter was set some 0.005" higher than the inboard. Thus the outboard cutter did a roughing cut and then the following inboard cutter did a finishing. Neat concept. Not too useful for little machines where every cut is a thousandth or two.

I made a large diameter flycutter to fit my Sherline:

It is 4" diameter and about 0.6" thick. The two 3/8" holes are for cutters and the 10-32 screws hold them in place. No plan to use two bits at once but for balance I made it symmetric. OK, OK, the whole truth. On the first C-bore the part slipped and the bore ended up "too far" in board. So I went around 180 degrees and cut one correctly. But if anyone asks I'm going with the balance explanation!

Here's the cutter in action, and the finish obtained on a small test piece.

I'm sure the cut and the finish are mostly controlled by the tool bit. I'm not great at off hand grinding tool bits so I'm sure better results than I show can be obtained. The "Tryally" vice was used to hold the test piece and it worked very well.

For the Travelling Show this flycutter is large and heavy. It didn't make the cut, but if I find I need it it could still be included.