I basically want to make a (portable) soundproof container that I can put my cellphone in so it can't pick up any and I mean ANY outside noises.

I have business meetings with people and I don't want to be recorded so I was thinking of making a soundproof container that they could put their phones inside instead of having them frisked and then asking them to leave it in their cars.

TL;DR how can I make a portable soundproof container or pouch?

Thanks in advance!