If you can butcher an old seatpack, it can be done much neater and easier.
These fixing are from an ortileb seat pack, and they can go back in if needed, they just screw through the material, and have a domed plastic hut inside so it doesn't snag anything, this is stuffed with an phd long and extra wide sleeping bag, so would easily fit a normal bag. I smeared silicon around the holes before attaching the fixings and it has remained waterproof.
Using the fixings means its very stable, I also added a webbing strap just to help support the weight.