looks like i'm going to have to buy a rack bag for my day rides the barley doesnt sit at all well on the narrow rack
I ran into this problem myself when attempting to mount a rack pack to my rando bike's Tubus Logo Evo rear rack. I just used a sheet of DuPont Zytel plastic intended for another purpose. I slotted some holes for the rack-pack straps and attached it to the rack with nylon clamps, buttonhead allen bolts and nylock nuts. Done.
Putting a stiffener in the bottom of the rack pack helps but does not solve the problem of Tubus racks with too-narrow tops. The real solution is to attach a platform of the same width as the bag. Mine sits level and securely with no wobble.
I'll bet Paul can find something for you.
All the best,
Dan. (...who can think of a variety of suitable materials that will do the job)
EDIT: I see you've no lower rail to mount your panniers, Anto, so you'd need to mount them to the top (only) rail. A removable platform or larger slots/notches might work for both rack pack and panniers.