Best external charging option?

Hey everybody,

After using my HTC One for about a week & a half I am still struggling to make it through a complete day without needing to charge the phone. The problem is I hate charging the phone for just 20-30 minutes and then having to unplug. (I'm a student) Also most of the seats aren't near an outlet so I have to leave my phone by itself against the wall to charge which is terrifying since it could either get stepped on or stolen. Thankfully I have managed my battery pretty well and so far it's only died on me once during the day but I am having to baby it a little bit which I do not like.

I was wondering if any of you have a preferred external charging method for your One? Also, will using an external charger like this in any hurt the battery life in the long-run as opposed to just plugging it into a 'real' outlet? I would appreciate any advice about a specific brand, capacity, type of charger, etc that you guys prefer. I really don't know much at all about external chargers so your help is greatly appreciated.

Note: I know the Morphie juice pack released today but since my phone is in an Otter Box I don't want to switch cases every time I need a quick boost in power.