I did that for a fermentation chamber, but later converted it into a kegerator. Held 2 5 gallons cornies and a 10# CO2 tank just fine. Basically, it's just a wooden collar that comes out of the front of the fridge. I just made sure the cuts were exact, and it fits like a glove.
Did you end up siliconing or taping your extension to the fridge?
Nope, my cuts came out just right so it was a very tight fit. I actually had to fit it on the fridge using a rubber mallet. I figured the contact was adequate, although a bead of normal window/siding caulking wouldn't hurt.
Fermenting: ESB Kegged: Extra IPA, Brown Ale, American Wheat, Blackheart Stout Coming Up: Dunkleweizen, 3C Pale Ale