Commonly available 150mm/6-inch triangular glass prisms are great for holding in front of a camera lens for in-camera special effects. However, they're difficult to hold and quickly get covered in fingerprints.
This build is a grip that goes on to the ends of the prism. In the first hold, the prism is gripped through the rings with fingers, allowing for a firm hold on the prism while it is moved around for adding light leaks or for reflecting what is to the side of the camera into the photo. It can also be held in front of, say, a face to reflect or double up things like eyes.
In the second hold, the grip is held by the textured area outside of the ring to keep fingers out of the shot. By holding the prism along its long-axis down the lens kaleidoscopic effects can be achieved.
See more images here: https://www.flickr.com/photos/captin_nod/albums/72157709418823347
Print the grip flat on the plate, slide it over the prism and tighten using the nut. Print out the plate component and place between the grip and the prism if you need a tighter fit.