The freezer is a common spot. Thieves do think of it because that's exactly where many thieves stash their own "extra cash". Former congressman Bill Jefferson of LA famously stashed his cold hard cash there.
If you have an extra stash of cash you don't want to deposit (wink, wink, nudge, nudge), and it's too much to stash in a ziplock at the bottom of a fish tank, go the extra mile and head to the attic.
If placing it under insulation is too obvious, tie bundles of cash to one end of long pieces of string. You should be able to find either gaps in the interior walls or around the fireplace where you can drive a nail to hang the non-cash end of the string. Dangle the cash down these gaps in the walls. 4951