The 5.11 COVRT M4 bag looks to be just a tic too short for your needs. That is, unless, you put on a PDW stock of some kind. There was someone making a stubby collapsible AR stock, but I don't recall which company it was.
I've currently got my SBR in a Wilson racquet bag. I have a 5.11 COVRT M4 Shorty in the shopping cart because it'll be getting a can "soon" (read: As soon as the F Troop gets off their thumbs); I'm just slow on the "Checkout" button.