Hi Chuck,
Truth is, the squares don't need to be identical in length, the real issue is that they be absolutely stable when pulled down to the table with studs, and short enough the facing head can cut your angle block clean without hitting them. They do need to be be as accurate as you can make them in diameter from end to end, and once you get the angle block brought up snug against them, you can rough tram across the top, so you've got an even depth of cut for the first cut.
The can also be used on a face plate and the surface of the angle block faced off, if your lathe faces square and doesn't have much tendency to face off with a concave big enough to hurt, or worse, a convex face. They don't even have to be the same diameter either, just straight, and cut off square to the limits of the machine. I've got a couple made from pieces of motorcycle fork tube, about thirty five millimeters in dia., ground for accuracy and chromed for show. They already have nice big holes in the middle of them, and they are ground to within a tenth of a thousandth round and straight and cut off pretty easily. you probably could get an old fork tube for nothing at a bike shop.
mad jack