I use those styrofoam coolers all the time. I have built
incubators using some of those cheap styro ice chests
and have hatched lots of babies in them.

They are not so air tight as the metal or plastic ice chests
are so they would work well for hibernation I'm sure.

Dont think you need a fan. When they are cooled, their
immune systems are low so I think cool air being fanned
on them might do them more harm than good. I use a
hydro/thermometer which reads both humidity and temps.

I would soak a paper towel then ring it out then put it in a
small vented container for a little added humidity.
Probably check/change the paper towel for continued
dampness at least once a week.

John