Dennis and I are suggesting better contact of the dry ice with the payload than might have been accomplished. (We don't know because you have not supplied that information)
As to the amount of dry ice, I have done a quick calc. based on a 12 inch cube (again, payload size is missing information).
Instead of me doing the middle school arithmetic for your students, I'd suggest you have them do this for themselves to determine if 8 pounds is enough to create a decent thickness (maybe an inch or more) surrounding at least 5 faces of the cube.
Surrounding the payload with a good layer of dry ice in good contact may not require the several hours of "soaking" I mentioned previously.