I’ve been down this road for the past few months and have looked at quite a few options.
My requirements are a bit different as I initially started out with a much smaller AR (5” barrel) split into the upper and lower. I eventually transitioned over to a Sig Rattler and decided I wanted to be able to carry it assembled and loaded, which lead me to Vertx. They do have a crap ton of options, but none specifically designed around carrying a rifle broken down. That said, a few strips of Velcro one wrap would work well to secure both the upper and lower as their “concealed carry” compartments are lined with hook Velcro.
The one thing that I’ve noticed with most of these packs is the larger the firearm, the less “discreet” it’s going to appear, IMO. It may not outwardly scream “firearm”, but with a long, usually more slender pack, there isn’t a huge list of items that someone might be carrying in there.
Guess it really depends on how strictly you want it to be in a “normal backpack” format. You can go from a fully discreet pack complete with bright “hiking” colors (Vertx), all the way up to an FDE soft sided rifle case (complete with MOLLE and Velcro everywhere) with some shoulder straps sewn on.