This case is useful any time you want to use a laser to point a GoPro, but it was specifically designed for building arrays of GoPro cameras to do time-slice (or "Bullet Time") videography. Bullet Time shots were made famous by the movie The Matrix. See an example Bullet Time video here:
For Bullet Time videos, you need multiple cameras, and each one needs to be carefully lined up to get a good result. For best results, you also need a remote trigger so that all of the cameras are in sync (otherwise, it is a serious pain to line up each video file frame-by-frame).
Here's an example of how a video looks without the sync cable and with slight misalignments:
... not bad, but not as amazing as the first video.
The back of this case is open so that you can attach a bullet time sync cable (http://cam-do.com/products/bullet-cable). Small tabs on the top and bottom of the case prevent the camera from sliding out the back.
If the camera is more than a few feet away from the target, you will probably want to just use a laser dot instead of a crosshair, because the crosshair is not very bright from a distance.
This case was designed with OpenSCAD, and it was successfully printed and test-fit on a Lulzbot Taz4. The case fit a Hero3 and a Hero3+. I do not have a Hero 4 to test with, but feedback would be appreciated. I am still printing the rest of the cases for my array, and my sync cable is still in the mail, but I will try to throw in a picture of my camera array and maybe a sample video of the results when it is all finished.