I built a wooden box (but it looks like crap!) with a hole in the back for a mini dorm fridge...I'm looking to get some insulation for it as well. I was looking at the 1/2in to 1in sheets to insulate it with. Not sure how well it will work but it will be nice to not have to swap out ice bottles and be able to accurately adjust the temp (once I get a ranco or jonhson controller...)
If you haven't built your chiller yet then maybe this is an option for you...I bet you could find a little fridge for cheap. Mine is an 18in by 18in square that I got from my brother-in-law.