I was contemplating the idea of getting one of these:
http://www.ebike-easy.com/products/B...PL-250-HT.html

Looks quite decent:
http://shop.leichter-fahren.de/image...mages/97_1.jpg

I would still not leave the bike with the battery unattended outside. You can force the battery out with a screwdriver in less than 5 seconds.