Antonio Banderas's guitar case from the movie would've weighed at least 50 pounds with all the hardware he had stuffed in there. You would've been noticably straining just to carry the sucker, and it would have to be some kind of sturdy to keep from falling apart under the stress of all that wieght. Also, the loose guns would be rattling around in there.
Now, for carrying a single long gun, the guitar case would be fine (pistols are more easily carried in a backpack and it's less conspicuous too). If i were gonna do an AR case, i'd get one of the stiff guitar shaped cases, and install some foam cutouts to keep it place (and help reinforce the case). you'd also want to do something about those flimsy hinges. a simple strap arrangement aroundn the neck and body of the case would prevent accidental openings.
now if i could choose any toy from the desperado movie, it would be the groin gun. Of couse, theres no way that the *testicular* revolver cylinders would feed into the main barrel. but it would work as single shot. besides, it's more of a conversation piece than anything.