After my fun with the cylinders, I decided to cut and fit the sound deadener onto the back of the firewall. It isn't attached yet, though.
I'm to the point where I want to rivet the firewall recess into place, but can't until I install the rudder pedals and their brace. If I'm going to install the rudder pedals, I should get the deadener installed while it is still easy to get to it.