I thought over rigging a leather horse scabbard for a while. I see them on eWhore in the $40 range. Instead I decided to go with the Kifaru GunBearer. It didn't arrive in time for my hunt last weekend so I can't say how it works in the field, but sure looks like what I need. Hope this helps, at least it's another option to consider, and looks faster than using a scabbard rigged to a backpack.
Below is the link to their site. https://www.kifaru.net/gunbearer_hunt.html
I have a Badlands pack that has a rifle holder built into it, it's nice, but it isn't designed to retrieve your rifle quickly for a shot. It's more for in the hiking mode when you don't plan on having to take a quick shot if you come across game while on the move.