Here's some Instructables

I used these lenses

15x is a bit overkill even with my Samsung Note, but hey.
I've made it in 10x and 5x versions (you just change how far the phone sits from the lenses). I think 10x is a pretty happy medium.

The cardboard versions work ok, but I do like using foam board. If nothing else, it comes in black. But I find it more sturdy and more comfortable as well. It's super easy to work with.