I bought 3 cheap smartphone lenses (fisheye, 0.67X wide and macro).
They can be attached to the iPhone's camera thanks to the magnet that they have on the base but I didn't like that I had to keep a metal circle glued to my phone all the time.
So I printed FishGee's lens clip and I tried it. I like the design because it works both with thin and thick cases, but I modified it in order to make it more functional.
I basically added 2 circles around the existing one.
One is deep 1mm, and is for the metal circle that came with the lenses; the second one instead is 2mm tall, and it's needed to keep the lens in the correct position (the magnet keeps it in place, but without these "lips" you always have to place it correctly in the center of the hole).