A lady at the post office trick: You can turn a flat rate envelope into a pouch to increase the capacity. Just fold the bottom both ways, and flatten it out from the inside, folding the corners and taping them up into a rectangular bottom.
Stash Cash in the Door? - 15 Secret Hiding Places That Will Fool Even the Smartest Burglar. you know until you put it all over the Internet and then everyone and their mother knows you have secret hiding places.