Hi All!
Casting about for a ready means to carry a lightweight camping kit (sub-kilo down bag, self-inflating pad, tent and footprint, my rain gear, Pocket Kitchen [meths stove, windshield, 2 alu bowls and lid with Reflectix cozy], some spare tights and light fleece top, and food, the lot totaling less than 10lb/4.5kg), I found my present expandable rack-top bag a bit shy on space.
I found an Ortlieb Bike Packer Plus pannier with the QL-2 hook system will nicely and securely cleat *atop* a Thorn Expedition rack, making for a 20l rack-top pack. It locks in place as securely as on the side, saves the weight of a second bag, balances evenly side-to-side, and doesn't disturb the handling with the light load I'll be carrying. The rack cross-members are the same ~10mm OD as the top rail and struts, so it was just a matter of adjusting the hooks closer together and setting the lower fins (I use two) so they catch the next cross-member.
Presto! Instant and secure larger rack-pack for free if you already own the panniers.
Best,
Dan. (...who -- despite all appearances -- can do "light" as well as "expedition")