I use a pair of vice grips to hold the carto, with carto wrapped in a small bit of paper towel. They don't have to be ultra tight -- I can slide the carto/towel in and out of the grips.
For me, dremel with cutting wheel, cutting a slot, is fastest/easiest/cleanest. It'll vary, depending on the particular tank, but I find a single slit cut at about 1/3" up from the bottom of the carto does well. The slit is cut horizontal, parallel to the bottom edge of the carto. I cut mine so that the final opening is about 1/8" side-to-side (I can see that much filler).
After one or two tries (use an old, beat, carto, for experimenting), it becomes really fast, no dents, just the slot.