I needed a portable kegerator for an outing a couple of weekends ago, and made one out of a 2.5 gallon keg, paintball co2 tank, some cooler packs, and a 10 gallon igloo cooler. It worked great. It got me to thinking about how to deal with a 5 gallon keg.
I made an extention collar from a sleeping bag mat (walmart closed cell polyolefin from sporting goods 8 dollars), some carpet tape to hold the pieces together (I used one piece on the outside and another part on the inside to mate to the inside of the cooler), some gorilla tape on the outside to give it some body, and some shelf vinyl from the dollar store to make it white to match the lid. Not perfect, but not bad....
Custom tap handles and lighted signs.