I have used 1/4" brass split/cotter pins (google 1/4" not 6mm).
The eye of the pin is big enough for snap links to go through. It is also tough enough for sailing purposes.
Simply drill a small hole, so that the pin fits tightly, and glue in with CA. If they go go all the way through a mast/boom you can trim the ends with pliers.
The smaller brass split pins you can buy for modelling are too weak for working models.