Re: backpack for horseback hunt
If it was me I would either buy or have a scabbard made that will fit your rifle. Because trying to carry a rifle on your back while you are on the back of a horse is not only awkward but unsafe for you and the horse. You will be paying more attention to trying to stay balanced than paying attention to the horse under you. You can try it but first I would try swinging a leg over and sit on top of a 6 rail corral fence for a while with your pack loaded like you plan to have it rifle and all and see how difficult it is with the fence not moving.
Good luck, Jason