One other alternative: I've seen a couple of guys load up a Smith bar, stand next to it, and shrug it with one arm at a time - although, I've never tried it myself.
Of all my body parts, my traps grow the easiest & most, so I only train them directly about once every three weeks on the seated shrug machine.
I may increase that frequency since I've finally begun adding a bit more size to my stubborn-ass shoulders.
I have a naturally narrow clavicle, and worse, my delts have always lagged behind my chest, lats, and arms.
Big traps on top of already narrow shoulders make them look even more disproportionately narrow by comparison.