If you have a mill and a rotary table making one should be easy as well. Possibly why they are hard to find commercially.

Martin C

It is easy enough to do a first pass of 90 degree steps with a square collet, the component just needs a rotation of 45 degrees within the collet to complete the pattern for an octagon on a second pass.

Brian

Edit    Why not go for a 12 step version with the hexagon collet block? Exactly the same logic applies.

Edited By Brian Wood on 12/05/2023 11:16:32

A semi-finished version is available here:

Indexing Plates The complete Kit

I have always liked the idea here:

Edited By DC31k on 12/05/2023 13:10:12

Imagine a square of metal. Superglue it any old how to the end of a piece of bar. If each edge of the square in turn is kept parallel to the mill table below it, the square cut on the workpiece will be correct.

Keeping it parallel is fiddly (as the distance keeps changing) so it is easier to keep one edge of the square vertical using an engineer's square. Sure, the stock of the square will slide back and forth on the mill table each time you turn the part, but the blade will always be vertical.

It is a good technique to know. If you wanted a seven sided thing, you can just print one out on paper, glue it on the end of your stock and away you go.