Cutting 6x6 slides

I collected two rolls of of 120 Fuji Velvia from the lab, and have been spending the evening mounting them in glass mounts, I have just slipped with the scissors and cut into the best slide on one of the rolls. Has anyone any advice for me about the best way to cut 120 film for mounting. I have a slide cutter for 35mm but have never seen one for 120.

I have used a small paper cutter in the past sitting on top of my light table to illuminate thru the transparency for alignment. I put plastic sheet on the cutter table to protevt the emulsion