I think hard fixing a metal box on the rack and then sticking a couple of these on the side may do it..
second that, say 2 hobbyking 8AH lipos in foam should be easy to carry,perhaps in a cheap bpm/20quid ebay citybike conversion, my sympathy, this kind of thing is a real heartbreakCwah- that totally sucks.
I feel for you for the work that went into your battery even more than costs.
As well as other solutions do you think a couple of small cheap lipo bricks could be used for those short distance social evening occasions?
And the big one used for commuting/touring when you can remove it and have under your desk
Just you might be ok with both weight and doormen with a professional branded pack. Especially if you keep a copy. of an RC hobbyist magazine in your bag as well.
And if they did get stolen it would be not as had a loss
i have one of those on an oxygen kit, it's great for weight distribution and simplicity, but only attaches with a little waterbottle bolts (mines vibrated loose a few times), in my neck of the woods i wouldnt leave a bike out at night with it (locals would take pleasure in ripping it off), but it does detach pretty easy for taking with