I recently bought an Alpkit Fuel Pod. Haven't used it much yet but it looks superb quality. It came with a single length of velcro fabric that I cut to the right length - more than enough to make it very secure on the frame. And because it's not integrated into the bag itself, I'll be able to replace it when it wears out.
https://www.alpkit.com/products/fuel-pod-medium
I also have one of the Topeak top tube bags and have found that excellent and will continue to use it - the Alpkit bag is just for those times when I need more capacity.
Obviously I get that you don't really want a top tube bag though, so ignore this. The Osprey is clearly more what you're actually after and although I don't know that particular bag, I do know that Osprey stuff is generally excellent - I have two of the larger Talon backpacks and they're great.