I used Google's .eps drawings in 123d to design a version of Google Cardboard that requires no assembly.
Just print and drop your lenses in (45mm focal length, 25mm diameter).
It's sized specifically for the Galaxy S4, but it may fit other phones. I'll upload the 123dx files later so you can customize it.
Also, I intend to update the design with a snap-on top piece.

I'm also going to make slots for the magnet toggle, once I get the magnets. :)