I managed to port and make a basic controller for my lunar demo on Cardboard. You can steer by turning your head/body, and you can accelerate with a button press.
This week I built 2 cardboard VR headsets. One was from Google, I printed out their template, carefully cut out all of the pieces using an Xacto knife, and in lieu of lenses, I figured out you can use the extra lenses for the Oculus DK2, you need to seat the phone a little further away, but it works great nonetheless.
I also ordered/put together a "dodo" case headset. This one was a pre-cut and came with the necessary pieces. This headset accommodates larger phones too (like the Note 4) so i needed this one to build my bigger project of an open source GearVR headset (coming soon). Besides holding a phone and having lenses, this little box also has a popsicle stick which pushes a lever inside that acts like a finger on the screen of the phone, so you have input while playing, it works reasonably well thus far, but next step is adding leap motion, just waiting on the adapter to show up...
See more of the weirdness under the AR prototyping/cardboard link above.