I'm certainly no expert, so I offer this for what it's worth: I bought my kegerator with a shank/faucet already installed. The hole through the door had a short piece of 3/4" PVC pipe inserted into the hole. I assume that's to prevent crushing/distorting the door, as well as maintaining a proper seal. Unfortunately, I cant tell you how well it's working since I am finishing the process rebuilding the whole setup.
Today, I got a brass pipe nipple to make a "Bulk head" for the co2 line, similar to what you would do for a cooler mash tun. I selected a hole saw that would accommodate the nipple, and I'll probably use a small amount of silicone on the washers to seal it all up. (for a 3/8" nipple, I'm using a 7/8" hole saw, it'll be to big, but 3/4 was too small and they didn't have 1/16" increments.) Hope this helps, Good luck!