I bought an icecube cooler for $20 and then made a styrofoam lid to let the airlock poke through. Filled with cool water and then put ice bottles in it and changed them every couple of days. Granted, is was in the winter, but I lagered my beer for 4 weeks at 34 degrees! If you can keep it in the basement at reasonably cool temperatures, that might work.
(Look in my gallery for pictures, if you want)