It depends on how thick your foam will be. I have not used the pro-hot knife but have used the Industrial hot knife. That could be too much for thinner (1/2") foams. You see - the knife is 4" long and the section that isn't embedded in foam heats up A LOT. For thicker foams that could be the ticket. Also, it depends on what type of foam pink/blue or white. The white doesn't need the Industrial tools as much because it's easier to cut. The pro-hot knife could be what you need but here's a better suggestion. Email Hot Wire. The people over there are really helpful and if you explain what you need they can help a lot.
If you don't want to buy a special tool always consider a jig saw or a Dremel. Yes, they make snow but cuts through the foam great as well.