It doesn’t matter how often Cobber refers to the sound deadening material a a dead dog, it always makes me laugh.
I used aluminium backed bituminous strip, usually used for repairing lead flashing, which is why I had some about the place. On top of that a piece of reconstituted foam, a couple of inches thick, cut to shape, obviously.
This came off eBay & wasn’t expensive in context, I couldn’t detect any difference between the dead dog & my amended invention.
At the end of the day all you require is something to subdue the drumming resonances, you still want to hear the exhaust note especially if you have shelled out for an example of distinction with a characterful tambour.
M