I also seem to remember reading retention is proportional to temperature at time of write. Ie, best case scenario = write data when drive is hot, and store in freezer. Would be happy if someone can confirm or deny this.
There are ways to keep water out of frozen/re-frozen items, of course, but if you mess up you have water everywhere.
Re-freezing is also critical, the container should contain no humid air when it goes into the freezer, because the water will condense and freeze as the container cools. A tightly wrapped bag, desiccant and/or purging the container with dry gas would prevent that.
Color changing desiccant is a good idea to monitor it.