Cylindrical square from a lathe tailstock.

[tonyfoale]

For years I've been on the lookout for a 1-ish ft long, 2-3" dia hydraulic piston at scrap yards for precisely this purpose.
Johan

Your comment reminded me that I happened to have a couple of 20 ton bottle jacks open. i measured them to check what the were like.

Click image for full size.

The item on the right is my new cylindrical square, the other two are from the bottle jacks. The longer one measured up with the ends of equal diameter but around midway was 0.001" large. The shorter one had a gradual taper of 0.003" over its length. Careful setup in a lathe or grinder would allow bringing them up good enough to lap to an accurate shape. Not the ideal donors but they are readily available. The shorter jack was obviously built to lower quality standards, with rougher machining and twisted gaskets etc.