For beveled cuts I have a gadget, flat on the bottom, sized to be easy to push with one hand, with a 45-degree slot into which you can insert and lock a straight cutter blade. you simply push this along the edge of a straight-edge, held in place with your other hand (or clamped down if you are nervous), and it makes great, easy bevel cuts on standard mat boards.
I've had it for years, so I can't remember where I got it. It's probably either a Logan or Dexter device, available at an art supply store.