Another suggestion: I glued reflective strips to an old winter shell jacket. You have to buy this special fabric glue at a fabric shop. So far, the strips have not fallen off. I've ridden with this jacket in rain and light snow, too.

I think this is a lot better than sewing since you won't have to poke holes in your backpack. I didn't want to poke holes in my jacket since I thought rain could go through.