I had the same problem. Go to HD and pick up 3 - 1/2" x 2" or 3" length nipples (black steel tubes with threads on either end). They are in the plumbing section. Pick up 3 bolts, nuts and washers too.
Drill 3 holes around the perimeter of the burner, put bolts through the nipples, through the holes on the burner and that will give you a larger circumference to rest your keg. Works beautifully and costs about $5. I'm not at work so can't provide any pictures or specifics on the hardware, but hopefully you get what I mean....
Fermenting American IPA AG 7-9-11