If you've got a bit of extra cash to spend, I suggest:
Buy a proper Music Keg if you can still find one (the keg is discontinued and even former reliable suppliers like Crutchfield have none left). The Music Keg package has everything you need to get up and running with your system, including the elusive properietary cable, which you will not likely find anywhere else. And don't pay more than $250 for the package.
Then as a backup, buy one of the Audi Phatboxes on ebay, for about $150. The Audi boxes can easily be transformed into Music Kegs by a very simple firmware flash. With the Audi boxes you also get a 20GB disk and a fast USB 2.0 cradle (as opposed to the 10GB disk and 1.1 cradle with the Keg).
This may not be the best solution, but unless you know how to make cables, it is the only solution.