It really doesn't take anything other than a common plastic shopping bag.
Just enough to keep any moisture laden air away from the plunger..

BTW, I just looked and I have a couple open (but resealed) tube in my
kitchen frig that must have been there for a year.. I can squeeze the side
of the tub and it is still soft.. I may have to dig a cured plug of 5200 out
of the tip of the tube when I want to use them..

This question came up about a year ago and someone suggested putting them in
the freezer.. I can testify that it isn't necassary and the stuff is ready
to use when you take it out of the frig. but might require some thawing time
if frozen.

Steve
s/v Good Intentions