Methanol, aka methyl alcohol, combined with water should do it, but you'll have to mix it right because as stated above, you want to stay away from pure methanol, as much as possible.
Here's the recipe:
http://www.overclockers.com/articles609/
ref page 2; looks like 50:50 will get you near -50deg C.
As previously noted, keep in mind that you'll have to keep your loop *sealed* to prevent vapors from escaping.