Perhaps the newer members of the Minox community do not know the name John Birkby, of FilmTech company in the UK. He invented (and produced) the smallest Minox film cutter, called Zipslit, something real simple, but it had a good run within the community for some years. I myself used it for some 8 or 10 yrs, heavy slitting 35mm to Minox format, and it stayed true and reliable. Keep it clean, make sure the blades do not rust or dull, and it will care of your films.
The size of the cutter is very close to that of a Minox film box, and it opens in two; it has 3 cutting blades fixed in what appears to be a hardened epoxy base, backed by hard black cardboard. Alas, this item is no longer made and it is unavailable currently. I still have mine, but the blades are somewhat worn and chipped. As the blades are regular cutter blade sections, I probably try to replace them, in the future. (Note: I have changed them in 2022, used 3 single pieces, taken from a regular cutter blade).
Disclaimer: I am no way related or connected to the manufacturer or seller of this item. I have bought this film cutter more than 10 yrs ago and I was (and am) happy with its functions. AFAIK, this item is no longer produced.