Just place the piece of foam on top of the bench, use a Sharpie marker to mark the edges, then use a kitchen carving knife (electric) or whatever you have to cut the foam.
We cut the foam to these dimensions:
– 72 inches long
– 15 inches wide
– 3 inches high
The only cut we had to make was to cut the foam piece in half lengthwise. It was already the proper dimensions pretty much. So we got 2 bench-seats worth of foam out of one piece.