CESSNA 337 3M-4014 Sound And Vibration Damping Panels.

[n86121]

Where you can make it fit, more is better.
Thicker is better than thinner.
Doubled layers is better than one.
Cover everything and place you can that will not need maintenance access later on.

You could approximate M2
by taking ballpark rectangular dimensions
of the overhead area and both sides
and the back panel too!

Maybe double that one for two layers if possible.

Then buy 25% more,
so you will have it on scene while the interior is out.

If you ang get more delivered quickly,
then order more and throw it in,
without losing too much time waiting for more to arrive.

From my design experience,
having more of whatever you MIGHT need,
already on scene when you are in the middle of it
is usually more important than losing/wasting time waiting to get more delivered.

As I recall it was not terribly expensive per sqft.
Especially when compared to my time and down time.