I got the sound deadening underlayment I ordered. Very nice 3'x2'x1/4" panels. I used a spray adhesive to attach to the inside shelter walls. So far, so good. Not including the door it covered about 50% of the interior wall. It's still loud inside but not nearly as loud. When tapping the outside before it made a very loud and sharp, echoing noise inside. Now the same tapping results in a dull-sounding "thud". The sharpness and echo is substantially reduced. I'm pretty pleased with the result.
https://i.imgur.com/6jbcpbt.jpg

