What are the best external backup battery choices for the iPod Touch 4G?

Hello,

I am so disappointed with the battery life on my iPod Touch 4G, even when I turn off location services and notifications and put it in airplane mode.

As such, I realize that I am going to have to get an external backup battery, even though I don't like the idea of carrying around yet another item.

What are the best choices out there, especially any ultra-portable (aka small and non-bulky) ones? I came across this Juice Pack Air - iPod Touch 4G Case, which seems like the perfect solution, but it is on back order and seems to have been so for a good amount of time. Who knows when it will be in stock again.

TIA for any and all help!