If you layer two pieces of MonoKote together, topside to topside, so that you cut mirror images, stick them together with temporary double-sided Scotch tape. It will hold the pieces well, but easily strip off when you're done. Put the tape along the cutline, and cut through both pieces of covering and the tape. That way, the covering won't move as you cut it.

Cutting on a large piece of glass also works very well, but it will dull the knife blade quickly.