If your cardboard viewer (or clone) does not exactly fit your phone, you can get some weird artifacts when viewing demo's/ apps (extra distortion / invisible screen areas etc..). 

Cardboard apps assume a certain geometry:

  • vertical alignment of screen and viewer
  • horizontal alignment of screen and viewer
  • eye distance to lens 
  • lens distance to screen surface

Since the last two are mainly determined by the geometry of the viewer itself (Ok, lens distance to surface also depends on the thickness of your phone...), these are usually OK, but the frist two depend on the dimensions (width/height) of the phone. 

I purchached a BroFish cardboard viewer (B.T.W: great value for money!) and it was obvious that my iPhone5 was a little too small, it sank too low and could wiggle from side to side.
Not wanting to use the small rubber band to hold it in place, I did the following: 

It came in a carboard box, how appropriate :-)

From the packaging box I cut out 2 identical frames.
Each with with dimensions of about a 1 cm (1/4 inch) around the perimeter of my Phone.
This was in my case an iPhone 5 which measures around 6.0 cm by 12.35 cm.
I made cutouts for the volume / power buttons, so they would not touch the frame when the phone was insterted. 
I glued the frames on top of each other with double sided tape.
The resulting frame I positioned on the cardbard back-plane so that it would be centered both horizontally and vertically in the field of view of the stereo lenses of the viewer.
Once satisfied with the position, I pressed it into place with some double-sided tape.

IMG 0694

Since I wanted to be able to test my App connected through USB, I also made a cut-out for the Lightning connector, so I can test connected.

So there you go!