Anybody using one?

I've enjoyed shooting with graduated filters with larger cameras, and am hoping I can do this with my XF100. Anyone have advice on what fits, what doesn't, 3x3 or 4x4, etc.

I'd like one with a rotatable holder, and the ability to slide a filter to adjust where the gradient on a grad filter falls. (maybe they all do this?)

Thanks

I'm curious about this as well.