Found a way to square up big frames without a helper

Last week I was building a garden gate for a client in Portland and the thing was 6 feet tall, way too heavy for me to wrestle square on my own. I tried the usual diagonal measurement trick but every time I tightened a screw it shifted a quarter inch. Then I remembered an old trick my dad used: clamp a straight 2x4 across the corners before you fasten, then check the diagonals. The clamp holds the frame in place while you drive your screws, and it kept everything true to about an eighth of an inch. Took me maybe 10 minutes extra on the whole job and saved me from having to redo the brace cuts. Anyone else got a go-to method for solo framing on larger pieces?