Been looking for a "non tactical" everyday walk through the parking lot with out drawing attention backpack for a shorty AR for awhile. I think I found it. Was in Home Depot buying plumbing parts for wifes new bathrrom project and spotted this. " Milwaukee Jobsite Backpack". 10.5" Upper and lower fit perfect. It has a nicely padded main compartment, second compartment has pockets that hold spare mags like they were made for them. Lots of misc. smaller pockets for gear. And best of all hard bottom and very well built. Was on sale for  $79!         https://www.milwaukeetool.com/hand-tools/storage/48-22-8200

Great find! Do you think a flush 16" barrel would fit at an angle?

My 10.5" upper with a AAC Blackout suppressor mount just fits standing straight up. I'm pretty sure a 16" wouldn't fit. You could get a 11.5" or 12" in at angle.