Sorry, I haven’t had that issue before… in a car or enclosure…
because at some point the enclosure becomes the building its in, and that’s the situation I’m in. I also don’t agree with noise suppression because it is how I know what it’s doing and if it’s in trouble when I’m chasing other squirrels in the shop.
I think foam egg crate can be another option, they use it in sound studios ( Amazon.com : egg foam soundproofing). granted the stuff their using is probably supper dense rubber like and not the same as a bed padding (thats what I used it for camping). I think maximizing the surface area is what ultimately mutes the sound.
My shop was supper echo-y. adding rubber mats on the floor really brought the decibel/echo level down! I’m glad I did the whole (95% of it at least) instead of following other advice here (Anti fatigue mats). I’m sure if you can figure out how to hang it, you can use the same rubber mats, or like, I got