FYI if you live in China, you can buy a kit (including machine-cut cardboard, velcro and lenses) for <50 RMB ($8). Magnets and NFC can be bought cheaply from other taobao sellers.

I made one by hand myself, and bought kits from two different vendors.

- the one I cut myself works, but it looks like trash and is not durable. Also, the lenses I used distort too much

- the one from the first vendor is great. The design has been modified so that the lens holes are a little wider, to accommodate the wider lenses supplied: http://tb.cn/qdMexXy

- the one from the second vendor was almost as good, but the doesn't have the slightly wider lenses/holes

If you're using an iPhone, I recommend the app '3dtube' for watching 3D videos. Also, search the App Store for 'durovis' to find other compatible apps.