There will be a limited number of FREE Google Cardboard kits so it will be first come first serve. Just bring a sheet of cardboard or any other sturdy material to create the box.
The only thing you need to bring is the material you want to use to create the viewer. Lenses, magnets and other parts will be provided FREE on a first come first serve basis. The most used material is thick cardboard, like the one that is used on pizza boxes. I went to my local pizza parlor and asked to sell me a box for $1 and they did! Others have used vinyl sheet or corrugated plastic usually available in a DIY Craft stores like Michaels. You can also use wood! Yes wood!!! I saw one made of Popsicle like veneer sticks! Whatever material you chose to use it has to be sturdy and hard enough to hold your phone when attached to it.
Here are some examples of Google Cardboard Viewers:http://www.google.com/get/cardboard/get-cardboard/#build-it
I you want to bring your own parts, this is what you will need. You can always substitute the materials you think will best work for you:http://www.google.com/get/cardboard/get-cardboard/#build-it