I've heard folks talk about humidity-sealed cases, but I don't think that'll help on a plane. See, the problem is the pressure - if your case is sealed and the air pressure goes down (as it does at 30k feet), the case would either have to vent some of its air out or explode! And once it's vented, you've got low pressure inside the case anyway.

Just in case, let the strings off so it's not under tension - the cold will cause the strings to contract slightly which may not be good for the neck.

Grab.