How to cut open cell foam

My local gym was replacing their padding and I picked some up for my garage wall. The dimensions of the foam are 4’x6’x18”. I don’t need an 18 inch thick pad to land on and I’m trying to cut it into two 9 inch thick pads but it’s proving quite difficult. Any suggestions on how to cut through such a huge piece of foam?

An exacto knife doesn’t do a great job, and right now I’m trying a bread knife while wedging pieces of wood to try to keep the two sides of the foam apart. 

Veddy, veddy carefully. . .