Rayguns are pretty cool: they have an immensely interesting and arcane look, but they're science-fiction enough to not be able to hurt anyone in real life. They've got style and flash. No retro-futuristic heroine or hero should ever be without one.
Disney licensed an inexpensive plastic toy "Sound Phaser" in conjunction with their movie Meet the Robinsons; it looks a lot like a classic raygun. In need of a prop for a "steampunk vs. cyberpunk" Halloween party, Craftster user TheBon gave a steampunk makeover to one of the raygun toys: after adding a paint treatment and gears, she has a neo-Victorian masterpiece to rival these $690 models from Weta.
If you would like to make one of your own, you'll need the gun itself, along with several kinds of spray paint (including a primer meant for use on plastics), painter's tape, detail paint, gears (clock parts will work), and a few other supplies. Can't get your hands on that particular gun? Here's a different execution of the same idea.