Sensei 77-82mm Aluminum Step-Up Ring

Omar, You can try a rubber band around the ND and flat pressure down into your palm in the other hand. Even flat pressure is the trick. You also can try a sharp edge of a knife (like from a weatherman tool). Get the blade in between the two rings and twist with subtle pressure, defiantly much riskier, but I've used both methods. Hope this helps.

The outer diameter of the Sensei 77-82mm Step-Up Ring is 84mm. However, I unfortunately cannot confirm if it would be directly compatible for use with the NiSi M75 75mm Filter Holder.

Yes, you can stack 2 filters with the same size thread but beware of vignetting (darkening of the edges) with wider angle lenses.

They are about 5mm thick which is about the same as a standard UV filter ring.

Yes, the black color and finish covers the entire step up ring.

The Sensei 67-95mm Aluminum Step-Up Ring has course 1.0 pitch threading on the 95mm side, and standard 0.75 pitch on the 67mm side. This is fairly standard for these sizes.

If your filter/attachment is larger than your lens, it's a step-up. In this case, your fisheye attachment is 37mm and your lens' front diameter is 30mm, hence, you'd need a step-up ring.

Yes, it's meant for any circular filters with 82mm diameter.