Don't know how anyone can answer your question without knowing the shape and dimensions of your backpack.

Another option might be to modify your backpack to let it work as a pannier. I added pannier hooks to the outer edges of my backpack. The hooks lock on the rails of my rear rack so it's very secure and doesn't need any lower attachment to the rack and the hooks are far enough out to the sides of the pack that they never touch my back when the pack is being worn. When I use the pack as a pannier I flip the shoulder and waist straps over the outside of the pack and secure them out of the way.