I'm sure there have been other posts on this, but hopefully this will help someone in a similar situation.
Here in ye ol' Portland, it doesn't get cold enough to use your basement for lager conditioning, so I was trying to come up with ideas to solve the problem. I plan on getting a hold of a cheep chest freezer and a temp controller soon, but I really wanted to get this beer conditioning now, since it is my first lager.
A while ago, I stepped up to 10 gallon batches and moved from my 10 Gal round cooler to a 15 gallon square one, so I had this spare MLT just sitting around. I initially just thought I'd put water in the thing and let the carboy sit in there with some frozen water bottles, but buoyancy decided that a 6 gallon better bottle displaces too much water and really wants to float, even with 5 gallons of beer in it. So, I cut a hole in the lid with a dremel, put the frozen water bottles in and crammed the lid down until I could turn it enough to lock. Instant lagering chamber.
The whole thing cost me $3 for 6 water bottles from Fred Meyer, and it is holding a steady 38 degrees in my 58 degree basement.