Radio/megaphone effect live?

[loveandbass]

Just use the real thing!

 

[mpdd]

the guy i work with in burbank is a producer and he has this vocal effect thing that can do a megaphone, but you probably don't need the other 250 things it can do

 

[Thor2]

TC Helicon has a box that has this effect (and many others) - works well.
Have also used a real bullhorn (looks cooler, but doesn't work if you need your hands free to play) - as others have mentioned, be sure to sound check it though as feedback can be a issue.