Are you dead set against a seat post bag? There are tons on the market and cheap ones too that have the proper steps and whatnot to stay securely under your seat. Or as suggested, just stick all this in a zip lock in your pannier.
Fannypacks... Again there are a gazillion out there.
Nope, i have a large seatpost bag (actually a cheap dry sack lashed to the seatpost with some custom webbing) for my sleeping bag and hammock.