Cylindrical square from a lathe tailstock.

[Frank S]

For a cylindrical square that is already very nearly +- .0000 in every dimension get your hands on box of new old stock racing solid lifters for a big block chevy Ford Chrysler they wont be the largest but at about .875 to 1" by 2.25" if you are checking something not very large you can't beat one of them an added plus is they are polished to mirror and the hardness will be off the books