In the past few years, we have covered plenty of iPhone rigs, lenses, and filters that let you get more creative with your phone. The NiSi IP-A iPhone Filter Holder is also worth a look. It attaches to the top of your phone and lets you use NiSi IP-A circular filters. You can stack up to 3 filters. With a square filter holder, you can attach square filters to your phone.
You can use this iPhone filter holder with ND, polarizer, and other filters. It should work with most phone cases. More information is available here.