With the Accsoon Cineye now available, it may not be practical given that you can just use your app on the same iPad that you can control the camera too, by just switching apps.
I've tried the Hoya (non Solas), Tiffen, Aurora PowerXND Mk II and settled on the SLR Magic. That filter does tend to warm the image so you should use the Image Enhancer filter to compensate.