Thanks for more responses/
[MENTION=11675]cliff[/MENTION] -- already have some Ortliebs, but want something theftproof, and also not so expensive in itself, kind of like the waterproof otter-clone box. There are some like that on ebay for ~$25 or so, too.

Locking those is a little more problematic, I looked at one of the HF ones, and you need two locks.

I've got one of the scooter cases on order and will give it a shot, $20 shipped. Disadvantage having the weight up high, not sure how well it's gonna be secure.

Advantage of the hard shell is that it's not clear if there's anything in there or not, something a ortlieb could imply otherwise.