cut a hole in the back of a medium to large sized plush ,the hole should be smaller,or the same size as the zipper you should have.
find a zipper.you can either remove it from an old bag,wallet,or whatever you want,or you buy one from any craft store.
no zippers?you can use buttons.but they arent really that awesome.
open up the zipper,and sew it to the openings.
sew a piece of fabric to the inside of the plush so that the stuffing doesnt get all over your secret stash.
remove some stuffing for extra space
Hide all your secret things,be it money,notes,trinkets, or whatever.