AA Battery Box

I redesigned this Battery box starting from the excellent job published by [Peterpur](http://www.thingiverse.com/thing:162966).
I did my new release of the battery box to improve two aspects:
1. applied round upper and lower corners as they resulted a bit too sharp;
2. applied a guide for rubber band to keep box closed even if carried in a mountain bike backpack.
I also modified external shape to follow batteries body, but it is just an aesthetic improvement with no other purposes.

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