Try a set of two 14-ounce discrete, reusable hidden flasks that look like cooler ice packs. You could use them for that, of course, but that's no fun. Or fill one up with that liquid that never freezes, and keep it cool with another. The opening is 1-inch wide, which means you don't need a funnel to fill it. The cap is water-tight and leak-proof. Price: $9.97.