Check out my new diggin tool! Saw it at Home Depot for $12.95 in the plumbing section. It's a Malco 6" Duct Knife, double edged, stainless steel blade with soft molded handle. Also came with a sheath.
Used to cut that rigid aluminum foil covered fiberglass duct work and tape for HVAC. These are VERY sharp and heavy duty. Has a wide, full length shank through the handle. I also bought one of these but haven't tried it for cutting plugs yet.