I have a
Altura Arran rack bag. It's great having loads of compartments to keep things seperate & big enough for full day rides, & maybe manage an overnight at a push. I asked at a LBS & the landing strip isn't available on it's own, so this needs to be moved between bikes. It fastens to the top of the rack with a couple of straps & is secure enough. The bag then velcro's on top & there's 3 (iirc) clips to secure it fully. It tends to wobble a little bit when loaded on the bike, but not that noticeable when riding.