Since the zipper pulls on most RS bags don't have a hole at the top of the pull and only a small one where the pull meets the zipper, not all securing devices will work well. I found a couple that do the job especially if you are trying to secure the zippers on a day bag.
The small size fits well. They are fairly easy for us to use but add one extra layer of work for thieves. They come in black, orange and green.
The other thing I have been using are
They take longer to put on and off but also work well. I like the ones that are slightly shorter but couldn't find the link. They are a little less noticeable but harder to put together due to their small size.
Some of you may think this is overkill and just trust your instincts. That's fine. Do what you think is best. But for those of us who want an additional layer of security, but not go too far overboard, these might work.