What about something like this
Adapter plate assembly???
If I did the math correctly, it looks like the plate is about 3-5/16" x 4-1/8" and should span your side by side dual master cylinder. You just mount it long ways across your chambers long side and it should properly cover the chamber and still fit without interference against the firewall, I think.
You could even fabricate a similar plate to specifically fit your application using common materials from a hardware store.
