On the Spanish YouTube channel, I’ll Show U How It’s Done, the guys show you how they made a pretty sweet Rick and Morty portal gun from little more than cardboard, reclaimed LEDs, batteries, and some EVA craft foam.
The video is in Spanish, but the YouTube translation is decent enough and the visuals tell you most of what you need to know. They also include a downloadable template for cutting the parts for the gun.
The only real expense here is the EVA foam. Everything else is basically trash. And if you wanted to skip the foam skin, you could probably get away with just painting the final cardboard gun with white acrylic paint.