Maybe you should cut a keg sized hole in the lid of an 10G Igloo water cooler and place the keg down in that hole. That should cover 90% of a keg at least and insulate your cooling water for less frozen bottle additions.
I did this for a portable kegerator, except with a 70-quart igloo ice chest. You can see what it looks like with a ball lock keg via my posts with photos on page 4 and 5 of the old thread on another board at:
http://forum.northernbrewer.com/viewtopic.php?f=3&t=77879&start=45FWIW, it insulates really well. I can keep the beer ice cold serving at least 2 - 3 days in hot summer weather, without adding more ice. With a keg inside it holds 40 lbs of ice, so plenty of room for frozen bottles to maintain a ferment temp with fewer bottles as suggested above.
In case you want to reconsider another alternative to the ferment cooling bag...
And it could do double duty as a portable kegerator / jockey box.