Topeak makes a bunch of quick release handlebar bags that have an extra mount for whatever you need. I use it for my computer, but H has used his for his light. (It shines over the top of the bag)

This is the mount showing the extra bar:


The bags come in a few versions, some with separate shoulder straps, some that are waterproof, many sizes, etc.

I have a small one that converts to a fanny pack (I've never used it as one), and it's plenty big enough for a camera, arm warmers, extra snacks....

It was like $25 at Nashbar when I bought it, if I remember correctly.