Usually something that has a clip for a bag or a jersey will fall off when clipped onto either, from bumps in the road. Fortunately they usually come with a bracket that is secure, mounts to the bike, and is a quick release. Most LED tail lights need to be aimed level and straight back to get good power aimed at the cars. Another reason to not clip to a bag.
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This will do what you want. I have one almost the same, but this one has better beam spread

http://www.cygolite.com/product/hotshot-pro-80-usb/