I've got a Pilsner in primary that I'm hoping to start lagering this weekend. I don't have a refrigerator that can accommodate the carboy, so I'm thinking about keeping it in a largish lidded foam container another, larger, carboy came in and swapping out bottles of frozen water to keep temperatures down. I'm not sure how futile that might be. The other option I'm considering is keeping it in my garage (in WI, so it's generally fairly cold). Does anyone have experience with chilling a lager without a dedicated refrigerator that can offer advice or other ideas?
As you may know or will soon find out its not easy maintaining a constant temperature when lagering without a refrigerator and controller. One thing that can make it easier is to submerge the carboy in water in an insulated vessel. This should slow the warming process so you can change out the ice packs on a regular basis. Another trick is to wrap a wet towel around the carboy that is in 2-3" of water and turn the fan on it. This can drop the temp about 5-8 degrees.
During the winter you could try keeping the carboy outside (in your garage) inside another larger container filled with water (as the other answer suggests). Then, assuming the average temperature in your garage is below lagering temperature this time of year, use an aquarium heater or two to try to bring the temperature up to ~50 degrees (or whatever your target is).
This link has a good estimating chart for how many degrees a given wattage aquarium heater will add to a given sized "tank". You might need to turn the heater up (or use two) on colder days and use only one or neither on warmer days, as the ambient temperature changes. For daily cycles, you could leave one plugged in all the time, and set the other on a timer so that it only comes on at night.