The trick is to use a long, sharp blade. The use long smooth strokes, pulling the foam apart as you go along. I f it's like 4" thick it will probably take a couple of passes. hold the cut open when making the follow up cuts.

Not sure if this is the solution you were looking for, but I've carved foam (several different types, including both open cell and closed-cell) with an electric turkey carving knife. It did leave riffles from the serrated knife edge, but they pretty much became part of the design. It works well because you replace the pressure of you pushing on the foam as you cut (which deforms the foam) with the sawing motion of the knife. Then you manage the pressure as you go.

I made some sea-horses- about 3' tall and 12" deep that we put on sticks and wired up with EL wire and mounted on the backs of our bikes one year for night-time friend finding on the playa at Burning Man. It was kind of like carving those wooden horses that you could get with hobby kits back in the day when I was a kid (aghh!).

have fun!
Chris

For cutting slab foam for furniture or bed use>>
I use a well honed 25cm blade Japanese sushi knife. ...a small little razor knife won't do it. First mark the line of the cut with a Sharpie. then compress the foam with a long preferably metal straightedge on the marked line. Then draw the knife along the aligned straightedge with a deliberate smooth consious pressure watching at all times to see that your blade stays precisely vertical. If necessary go back and shave off some areas of the resulting cut that may appear higher until it looks about right.

For cutting sponge, natural sponge, as used in the bath..>>
first soak it, then squeeze out, then freeze it, then take it out of the freezer and saw it to shape while it's still hard.

What blade pitch - what speed?

I have images of the foam being dragged down into the guides below the table.