I’ve adapted a 3MT one very similar, from Chronos originally I believe, to a parallel shaft to fit W20 collets. I found the unwanted 3MT arbour could be unscrewed. Made a parallel shaft from some 20mm steel and screwcut the end, 16mm if remembering correctly, the flycutter was about 20 years old or so, so worth checking if they are still made with a threaded arbour. some I’ve seen at shows have a roll pin to hold the head on, so guessing a parallel fit with maybe loctite but still adaptable I would think if you can’t find a commercially available one. I was concerned that with the short gripping length of a W20 collet the cutter might slip, but despite sometimes being a little over enthusiastic with feed rates and depth of cut before I got a carbide face mill, nothing dire has ever happened, also used same in ER collet with no issues.

As well as a big 2 1/2″ one which seldom gers used I also had one of the small sets of three, bought about 30yrs ago and they served me well until I went over to the facemills. Still handy for the odd job or when using a shaped cutter to profile an edge.

Also a nice weekend job to make one yourself.  Keep the cutting edge on the centerline by ofsetting the milled grove. The three on the bottom are homemade for varios jobs. You can use almost any lump of metal you have and decide what shaft dimensions