Cylindrical square from a lathe tailstock.

[Frank S]

As long as you don't need too many zeros to the right of the decimal point or a high Rockwell hardness new flat or dish top pistons can be a good make do a hole can be drilled through them for rigid mounting the diameter to height ratio is in reverse though but as an added plus by using the gudgeon pin holes 4 of them can be used to make a small lathe or a centering device or many other things.