Sailor sells empty cartridges for syringe filling. While this sounds completely pointless given you could just buy one filled with ink and then use the empty I believe they are made with more robust plastic that prevents stretching at the neck after several uses and the potential for an unexpected leak or excess flow at the nib. Something to look into.

It's shitty that we need workarounds like these. I have a Platinum Century that I bought a converter for, and the converter is juuust loose enough to be an issue. If I'd known that a converter wasn't going to fit properly, I'd have just gone straight to an empty cart, which fits perfectly!