I made a refrigerator that fits nicely in a hole I cut out under the stairs (so I could mount 2 taps on the wall). At some point I may actually write a how to with pictures and all.. it was a real PITA but a lot of that was due to the confined space I was working in and the lack of any room for error.
Here's what I did:
build a box out of stryofoam (I used 2" thick extruded polystryene - about $11 for a 4X8 sheet from HD). Use wood to reinforce it. The interior walls and ceiling can be stryofoam if you like but I think you'll want plywood for the floor to prevent any scratching or compressing of the stryofoam. I made a plywood-foam-plywood sandwich for the floor and treated the wood with water-protectant. At some point I may put glass down as it would certainly work better but I don't have the funds for that at the moment. I also put a wood block in the middle of the floor to help prevent the stryofoam from compressing (though it is very resistant to compression).
Here's how I keep it cool:
Strip the innards out of an old junk fridge if you got one. I managed to get the whole shebang out in one piece without crimping any hoses. It consists of a compressor, a high pressure (exterior) radiator, a long copper pipe that ends at the low side radioator (inside the freezer behind the rear wall). They'll also be a fan, a timer, and a thermostat in there somewhere. Mark all your grounds as you disconnect them and then wire all grounds together and hook them to the ground in your outlet box or wherever is convenient. Mount the low side radiator inside the box and place a drip tray under it. I stole the powersupply from an old pc to power some cheapo case fans to circlate air around (I mounted them behind the radiator to blow the cold air out toward the kegs). These two case fans run all the time (I have them on switches so they can be turned off if need be). I also bought a rather expensive ($20) ac powered fan from radioshack to wire into the fridge's wiring in place of the old fan (it was designed to suck air and the fins did a poor job of blowing air when I reversed the spin of the motor, plus it was a pain to mount). This fan, like the original, turns on when the compressor kicks on. Remember, no matter how cold the radiator gets it will do a poor job of cooling things if you don't circulate the air around it.
I then built a compartment in the makeshift fridge to hold two 6.5 gal carboys and, just like in the fridge, built a little door that I can open or close to regulate the temp in the fermentation fridge.
Caulk it and use weatherstripping to seal any doors (the face and lid both come off of mine).
If you want a more detailed how to then I'll try and write one up. Of course the easier solution is the old fashioned one - use a block of ice to keep it cool. But you can't do any lagering that way.