A combination of a four-bladed easel and cutting down paper.
I cut 11x14 paper down to 11x11 and print negatives from my Rolleiflex at 10x10.
I take one sheet of paper, cut it down, develop it, unexposed, then dry it and flatten it. I put that sheet of paper in the easel and center it up the way I like. Then I take a fine-point sharpie and trace the outline of the blades onto the paper. On the top corner, I write the "FOCUS SHEET - 10x10" or whatever size I am making.
I make a focus sheet for every size that I like to print. When I change sizes, it's a simple matter of pulling out the focus sheet for 8x10, 4x5, 11x14 or whatever size, put it onto the easel then move the blades until they touch the outlines drawn on the paper.
If you use red ink (or whatever color matches your safelights) the writing will be virtually invisible under working conditions so you can use it for composing and focusing without being bothered too much by the writing on the paper.
My hardest problem printing square pictures doesn't have much to do with technical stuff. It's because I have a hard time finding square mattes. They either have to be special ordered or custom cut.
(One of these days, when I get rich, I'm going to have to invest in a matte cutter. )