Loyal D+R reader Scott pointed out that it would be easier to replace the old fluorescent tubes and the leaky ballast with a pair of compact fluorescent bulbs. A good idea! As the sign will be a nightlight for the shop, I don't need the light output of two 24" tubes and it will be easier to maintain in the future.
As you can see by the smoking shadow of the old ballast, the guts of the sign have been stripped and one of the new porcelain sockets is already in place. After drilling a small hole, the two sides of the knock-out punch sandwich the sheet metal. The center bolt is tightened to pull them together to leave a very clean hole. At $50-120 a piece, that tool box contains about $1200 worth of knock-out punches but, thanks to garage sales, I bought the whole lot for $40.