Someone mentioned hollow trees as an alternative to carefully camoflaging
boxes. The only problem here is that animals also use hollow trees as their
homes and hiding places, and are not too keen on funny plastic things
blocking up their doorways.

Sunday I was out trailrunning with the hound in an area where I've got boxes
planted, and decided to check to see if they'd had any hits. The one box I'd
hit in a hollow at the base of a tree was sitting out on the ground smack in
the middle of the trail! Fortunately, it's an unmaintained and extremely low
usage trail (a few fourwheelers on the weekends, but almost no foot
traffic). I think one of the fourwheelers had inspected it, because the
ziplock was unzipped, but it was otherwise in fine shape, so I rehid it,
this time under a log rather than in the hollow of the tree (note to
self--fix clues on website!)

I think a lot of the time we are quick to jump on "vandals" for causing
boxes to go missing but fail to recognize just how curious and/or annoyed
the furry woodlanders can be. I say this also as someone who has watched
entire cardboard boxes on her balcony disappear from around their contents
over the course of the winter, as the persistent little squirrels chewed
them up and dragged them off to their nests. I'd be curious if any of the
burlapers have seen evidence that squirrels chew on it? I ask, because one
of the other things they ate over the winter was several macrame plant
hangers. Not just nibbled or chewed, but shredded into a heap of tiny
fibers.