I've brought aseptic milk on the plane in the past -- it's supposed to taste better when chilled, but it's totally safe at room temp. Not sure what they have in HK in terms of box sizes, but for your return trip, you should be able to find 8oz aseptic single-serve boxes with straws in most grocery stores. Organic Valley in particular makes whole milk boxes (available at Whole Foods); Horizon Organic only does 1%. Stay away from the flavored milks in the US, as they have buckets of sugar added to them.