If you really want to use the Juice Pack Air with your iPod touch, then you could probably put some stuffing between the case and the touch to make it fit.
Bear in mind, that the touch is not only thinner, but shorter than the iPhone.
Dimensions:
- iPod touch --> 4.3" x 2.4" x 0.33"
- iPhone --> 4.5" x 2.4" x 0.48"
So there is a height difference of 0.2" and a thickness difference of 0.15" that results in a volume difference of 1.78 cu inches which is quite a bit considering the size that we're talking about.
To put that in perspective, the volume difference between the iPod touch and the iPhone is about half of the volume of the iPod touch itself. So I think that it will be hard to make it work. YMMV.
What about an external battery pack from a different vendor such as
Richard Solo?