[ALS]

Actually a thermos does work, but you have to be veddy careful transporting and it won't last long, so use it quick!

Small chance they'll let you walk out with a thermos full though.

[Jubal Harshaw]

Not only do you not even need a thermos, but the small size of the thermos might be a problem. I used to make ice cream myself (as per the article above), using a styrofoam cooler of liquid N2 from my lab. The cooler ($3.00 at any grocery store, put one inside another for more insulation) held 2 1/2 gallons, which started to evaporate immediately, but would take hours to completely sublimate off. Of course, I was working in a place that bought N2 by the truckload, so a cooler of N2 was only a few cents.