Jeremy:
Do you have a mat cutting machine, one where the blade rides on a hold-down bar, lik a C&H or similar type? If so, you can cut film with absolute precision and safety using the straight cutter side (not the 45 degree bevel side) of the mat cutter. The hold-down bar keeps the film from slipping, the sliding blade holder can't scuff the film, and the resulatnt razor slit is just as clean as a factory cut. In fact, the slit is a whole lot better than a standard paper trimmer cut.
Another advantage is that you don't need to thread the film under the hold-down strip on rototrim type trimmers, (where there's a potential for scuffing). Another, little recognized disadvantage of shear slitters, be they traditional paper trimmers, or rotating blade types, is that the unsupported slit edge of the film (the one that is pushed down by the upper cutting edge) has more potential for damage such as emulsion cracking or chipping along the slit edge.
With these tidbits in mind, it would be fairly easy for a mechanically inclined person to make a simple dedicated film slitter that uses a hinged hold-down bar, a film width locating stop, and a simple razor blade holder that slides along the bar to slit the film just as precisely as store-bought film.
One last thought: if you decide roll your own; get a pair of anti-static gloves, the kind used by people who assemble and handle circuit boards. Keeps static and dust bunnies under control.
I slit 8x20 film down from 16x20 and 20x24. No problem.