I think a 14" square would be about perfect for a keggle, especially if the inside supports are flush with the top of the square frame. Draw it out and you'll see the overlap on every side I'm talking about.
Try a big hunting, fishing, camping outfitter (like Wholesale Sports out west, or Bass Pro Shops or LeBaron's out east). They usually stock a great selection of turkey fryers and burners of all size and description. Oh, Canadian Tire also sells them.