Use small aero wedges. Had the quick link version first but the side of the plastic slider failed so switched to the strap version. They do wear out. I've had 3 altogether. The middle one had too much elastic on the seat strap so I sewed it into a loop, the latest one had a much shorter ekastic section. I've shortened the straps to fit and prevent short rub. I have a carbon seat post so I protect it from rubbing with a turn of duct tape, also I've learnt to put a turn on the inside of the stiff bit of the seatpost strap otherwise eventually the exposed stitching will wear through. I've had to restitch the top straps so I also reinforced them with glue. I've found the mesh pocket is gets holes in it quite quickly from being rubbed by the multi tool. It does sit on my only bike so does all rides, daily commuting in all weathers and longer rides.
Despite all this It's a good pack, fits two tubes, multi tool, tyre levers, other bits and bobs e.g. quicklinks, lump of tape, cable ties and can also take bunch of keys a gel and bar if squeezed in a bit.