At work, an electronic component we use suddenly started arriving in individually-sealed plastic bags, rather than bulk packages of 25. I guess the manufacturer felt that their product was too simple and convenient to use as it was, or perhaps they had a waste-production quota they were struggling to meet? So, I designed this little tool to slice the bags open. It clamps onto the edge of a table, up to 1.75" (45mm) thick, using a 1/4"-20 bolt with hex nut, and accepts an X-Acto or similar knife blade through a slot in the top to do the slicing.

If there's not enough friction to hold the blade in its slot, you can glue it in place, or wedge it in with a shim of some sort. Skirt material peeled from your printer's bed would probably do the job.