Had the same problem about a month back....
I used one of the multi-cutting tools from H.F. and cut out the hole similar to what Heater treater shows.....
I did not fork over the 120 bucks though... a piece of 1/2 aluminum tubing, some two part epoxy and a couple small pieces of fiberglass cloth, and voila..!!
I used a piece of plexi and some of the foil a/c tap to fill the hole.