Don't make it.. Buy it!
Dry ice for use in cloud chambers
"We were demonstrating cloud chambers at the ASE Conference, and needed a supply of dry ice. Rather than transport the means to make to make it, we bought some from www.chillistick.com, and had it delivered to the conference. The service was excellent and the polystyrene container in which the dry ice was supplied kept it useable for two days. We imagine that, with care, it would be possible to keep viable dry ice for perhaps as long as 4 days in a school, especially in winter.
Our advice is therefore: don't make it, buy it. Contact CLEAPSS for more information and guidance and refer to CLEAPSS Handbook Section 11.2."