How to get a marvin the martian ray gun?

For halloween this year, i am gonna be marvin the martian (big shocker with my user name and all lol) I know i am asking way early but i like to plan things early. I pre ordered the costume i just need a gun to carry around with me. If you could tell me where to get or even how to make a marvin the martian ray gun that would be great. Thx.