The cone has a base cylinder 25mm diameter 5mm high with a shaped cone atop it 20mm high tapering to 8mm. The whole thing has a 7.873 mm hole down the middle to take the 8mm rod turned down to a tight fit. (I have a 7.873mm rotary broach - found in a drawer in a vacated industrial unit).

The fun, however, is to make them myself. As well as my Amadeal 290VFF, I have a Clarke CL300 which would be an interesting base for a CNC conversion (Ummmm!) - there's another project I will never get round to.