I think the overall idea is carrying a gun, rifle probably, without looking like you have a rifle.
Try tool box carry.
AR-15 and the Maryland AR-15 web sites have two different toolbox conversions to carry an AR-15 A2 or an M-4gery. It even leaves the top tray in so you can toss a few wrenches, pliers and a screwdriver or two.
With a little ingenuity you can fit in three or four magazines too.
I don't know the law in other states, but in Illinois a gun must be carried unloaded and encased. There is no such thing as "in a gun case" rule. The toolbox passes muster as encased, and you have to break the AR down into its upper and lower to fit it in, so technically it's dissassembled and inoperable as stored.