/cgl/ - Cosplay & EGL

I'm helping a friend with a last minute cosplay and wanted to help him do King from OPM. It'll be subtle since he'll be in disguise with a mask covering his face with a hat, but he'll be walking next to me and my friend who are doing 2 of the other S rank heroes. I've been racking my brain trying to figure a good way to emulate the Kings engine sound. He'll be wearing a small back pack so I was thinking maybe I could have some kinda device that plays audio through a bass speaker that he can activate remotely. I'm just not quite sure the best way to go about it. Sorry if this is more of a /diy/ question. I'm open to any ideas.