For the best cutting, you can use a hot wire cutter (<--best), or a long razor. Since I did not have access to a hot wire setup, I used a thin snap razor cutter with a fair portion of the 'snap blades' extended. Small, thin, and very sharp makes nice smooth cuts removing the jaggedness you'd get from a normal knife, or the multiple scoring marks you'd get from a blade that has to use multiple passes for the cut. That super sharp thin blade also make corner cuts very smooth.
Using your off hand to keep the split foam lined up (and importantly) taut to the cut angle, gives much smoother and linear cuts.
Hot wire cutters give the absolute best results, but I've never had one. Had a buddy that moved that had a flat bed hot wire cutter and that was awesome. There are some companies on Amazon that sell hot wire foam cutting pens. But I've never tried one of those.