There is a stock knock out plate near the master cylinder on my Mustang. The knock out plug was removed when I purchased the car. The shape and the size of the hole are unique and I've not been successful locating something to plug the opening.
For the past few years, I've used a cut up rear shock plug. It is too tall and too wide to do the job but was better than nothing. It would have been best to tack weld in a piece of metal from the inside when I did the work under the firewall. However, I did not even consider it at the time and the inside portion of the firewall is now covered in Dynamat.
I decided to make a metal block off plate to fill the hole. The front and rear plates are made out of 16 guage scrap from the metal plate I created to separate the rear seat from the trunk area. I then cut out an oval piece of metal which fit about the exact hole in the firewall. The two 16 guage pieces are going to sandwich the oval piece in the hole.
I used RTV to somewhat hold the oval piece on the front piece of metal. I ran a little bead of RTV around the side of the oval and smeared it out rather smooth. This will serve as a little bit of a gasket. I then tack welded nuts on the back piece of metal to make installation simpler. I picked up some nice stainless steel screws to finish it off.
I worked on this little project for a few minutes each night this week. I painted the part and put the RTV on yesterday. Now that the RTV has fully dried, I installed the block off plate. It seems to fit well and should do the job nicely. It is certainly better than the hacked up rubber plug I was trying to make fit.