As I am sure you know the secret to keep dry ice frozen is to protect it from AIR. Other ice ("wet ice") and a cold environment
are not necessarily needed to keep the dry ice from "melting" .... you need to keep the dry in an insulated area and protect it from air. It will absolutely freeze items and keep them frozen for a good while (7-10 days). The less air exposure will lengthen this time.
I suggest pack your items pre-frozen on dry ice and pack them for the late phase of the passage
. Pack them in an inexpensive styrofoam cooler and tape it closed (airtight). Keep it below and do not open until you need these provisions and then try to open the cooler only once daily once you start using this supply. Captain
Bill is right .... large blocks last longer.
Reminder .... this is frozen CO2 and will burn unprotected skin .... wear fabric
gloves when handling dry ice.