If it's close but somehow not right, it's better to start over. Somehow, this is faster than trying small but vague corrections. With practice you'll learn a sense of your art snapping into square. Go get your roll of film, pop open the camera back, and accidentally kick the tripod. Welcome to the world of repro.
Next time load the film first.
Then bring the tripod to the camera. The longer the lens, the easier things are to square. If you're in this for the long haul, invest in a macro lens of 100mm or thereabouts. If you can't get it square, your canvas may not be square. Take both diagonal measurements.