You might also consider a leather or heavy nylon hanging garment bag (so it looks like you're only transporting a suit or coat in and out of your place). Obviously, if its the fold-over type, don't. Just carry it by the hangar at one end so you get the full length of the bag for your weapon, and keep it straight so there's no telltale bulges. You probably don't have to worry unless D-dawg is going for that clean cut, conservative, suit & tie look lately.
Another possibility is a large art portfolio such as those sold by Pearl Paint. They max out at about 3-4" deep, but would be good for transporting a relatively thin rifle or a few handguns. Of course, it looks like you're just another poor starving artist or art student schlepping your worthless still-lifes around town. Unless D-dawg happens to be an art collector, you'll probably be fine.