There are straps with a male buckle that are attached to either side of the bottom of the pack which should 'hug' the sides and attach to the female buckles which feed through the saddle rails. These need to be cinched down tight to compress the load and minimise sway
Probably the same way I messed up the Vischacha. You can put the saddle straps into the lower clips, whereas they're supposed to be for the roll closure.
It looks to me like the straps go over the rails. They're raised, so something is holding them upwards (https://lfgss.microcosm.app/api/v1/files/ab4640cff3c4e50909f91eba3e609939be8b5aa0.jpg)