I am also working on this problem. I have a battery manufacturer looking into making these. Once the prototype is done and has been tested we will let you know.
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you might take a look at doing something different..like making a snap-in battery pack that takes AA or AAA or some other commonly available battery. I was planning to look into this myself, but you are in a better position to do so, if inclined, I think.
a pack that takes 6 AAA might not stick out very far....and would provide plnty of cheap power. The design could easily be applied to make clamshell battery packs as well.
given a AAA is @ 850 mAh... that is a total fo 5100 mAh per pack... though I guess some useful power will be lost in the voltage regulation, etc.
Long life and off-the-shelf batteries would be a real help for travelers, I think.