an even easier and cheaper idea...
hardest part would be fastening the mat cutter #12 blades securely, and pulling the canvas through at an even -smoothe pace.
A leader strip and pulling the canvas from the two edges at the same time, or even attaching the leader to a strip of wood or roll core to pull it through, should work as long as there isn't too much gap in the slot. You could practice on a dummy roll of kraft paper.