Google Cardboard is an insanely cool idea. Make VR goggles with your phone, and its so easy! You've already got cardboard, you can easily scrounge some velcro and magnets, and....
Maybe you should have gone to I/O after all. Where are you going to get those lenses? 40 mm focal length? How do I figure that out?
Actually, you may already have them.
As you may remember from setting things on fire when you were smaller, to focus the rays of the sun you need to adjust the height of the lens over the focusing surface, be it an ant or a piece of kindling. In truth you are projecting a very small image of the sun on the paper, just like Google cardboard is going to project a very small image of your smartphone screen straight into your eye.
To get that focal length, just get out under the sun and measure how far over the focusing surface you have to hold the lens. That's the focal length. (simplifying slightly, but with the sun it's not too likely to matter).
The lenses that Google suggests are 45 mm lenses, which is about equivalent to a magnifying glass with 5x to 7 x strength. Some of the cheap jeweler's loupes are around this strength. If you're a teacher like myself, you might be able to use the strong biology magnifying glasses, but the lens sets are likely to be useless.
Alternately, you might be able to use the lenses from a pair of cheap binoculars. Mine, however, are too weak. The shorter the binoculars, the better this is likely to work.
In both cases you might need to fiddle with the lenses somewhat since they are almost certainly different from the Google ones. You need to ensure that the lenses are perfectly flat with the screen so they don't project the image into the wrong place. And you need to make sure that the distance from the lens to the screen is exactly correct so they aren't projected out of focus. I'd suggest thin pieces of cardboard to lift the lens holding piece to the correct position.
In addition, the Google lenses have a 2.5 cm diameter. You might be able to make other sizes work, but it might be difficult.
Most of this advice comes from the Cardboard & VR Developers Google+ Community.
Other key advice from them would be to change to mounting point for the magnet, as many phones mount them in different places.
Finally, I have been looking on the Chinese sites, especially Taobao. This seller is offering them for about 10$ US, claiming delivery around July 6. He claims that 40mm lens diameter will offer a better field of view - of course he may just have a good source for 40mm diameter lenses.