I have both the polar pro and the freewell variable. I bought the freewell because I thought that it would be more convenient than swapping the polar pro's depending on the lighting, WRONG! The freewell works OK but it is heavier than the polar pro's and the camera drops face down when attached, It does not cause a problem when the gimbal is powered though. The main problem with the freewell is that it is wider and the gimbal guard does not fit on the gimbal with it attached, so i was back to taking it off and on like the polar pro's. According to other experts (I'm not) the fixed filters generally are better optically than the variable filters.
You can use the cam-holder with the variable freewell ND filter, it doesn't fit perfekt, but it holds and protect the cam.
But if you wanna use the selling foam for the H with the frewell ND you have to cut of a bit of foam to gain space for the holder.