If you did check it, I would probably use those breather bags. they are made to be completely filled with water, with no airspace, so that would eliminate the pressure problem...water doesn't expand and contract under pressure. If you boxed them in a styrofoam box with a few heat packs I bet it would stay warm enough. And fyi, those breather bags are meant to be used single-bagged only, double-bagging them reduces or eliminates their ability to "breathe".
