The cardboard box needs to be large enough for a player to poke in his head. Face the opening away from the audience. Set up a striped pole and use a few other barbershop props. The "customer" wears a tight-fitting light-colored bathing cap to hide his hair and, over the cap, a long scraggly wig loose-fitting enough that he can shake it off when he needs to but well enough anchored that it won't fly off too early.
Barber is on stage. The customer enters and asks for a haircut. Barber checks him out, announces he thinks this is a job for his brand new haircut machine and convinces the customer to try it. The customer sticks his head into the back of the box, and the barber turns it on (sound effects). The customer yells, flails, flops, and goes through incredible contortions, shaking off the wig in the process. Barber, unperturbed, turns off the machine. The customer pulls out a "bald" head and races screaming off stage.