You can use a small ("Half" A4 size) paper guillotine - but you must be very careful. These guillotine-trimmers have a stopguide which you can mark the 9 or 12 cm fields so that you can cut film in the dark. Batch cut 4x5" sheets into 9 cm wide sheets first, then box film to turn on lights and adjust the cutter for trimming the 12 cm dimension. This was how I did it.

BTW, make sure you cut the side opposite the notch. You can feel this in the dark.