I've thought about making something like this myself and I would definitely purchase one if it were available. I wonder how to make an acoustically insulating

I've thought about making something like this myself and I would definitely purchase one if it were available. I wonder how to make an acoustically insulating phone case that is not so bulky one has to keep the phone in a bag. Acoustic insulating materials I've seen are fairly massive or have large spatial extent (or both).