Google Cardboard. It’s a template for turning an Android device into a cheap home-brew VR viewer. It’s supported by an SDK for tracking and outputting distorted video for the lenses.
And suddenly there is a shortage of 25mm diameter, 40mm focal length Plano-convex lenses in the world. 
Call me when they are selling pre-cut cardboard and lens kits. No way I’m cutting that out myself.