Totally agree, that plywood trick is a lifesaver. Had a client last month who wanted a huge basketweave border. Spent an afternoon cutting a guide out of some scrap OSB. Made laying out the whole section so much faster, just had to line it up and go. Saved us from constantly checking the angle with a speed square.
Scrap OSB for a template? That stuff would warp on me in a week. I'd be stuck re-cutting the whole guide halfway through the job. You must have gotten lucky with the weather.