Drwmmel would be by far the best. You cam get one at your hardware store for not to much money.
Also you could try heating up the knife. When I was at my hobby shop a bit ago they had a tool that could come in handy. It was like a soldering iron with an exacto blade tip. It was made for cutting foam and plastic parts so it should work. It was 30 watt so it should get plenty hot enough. Best of all it was only like $8.