Hardware search: Rotating step ring for mattebox use with rotating lens fronts

[QuickHitRecord]

My mattebox is mounted to a rod system with a sliding base from LockCircle, so lenses that extend when focusing or zooming are not a problem (the mattebox slides on the rods). But lenses with rotating fronts still pose a problem and I'm trying to find a some kind of anti-friction or bearing-based intermediary ring to put between the matte box and the lens's filter thread.

Before anyone suggests it, I know that I could remove the glass from a circular polarizer and use that, but I don't think that it would be smooth enough. I'd like to see if there is anything that is specifically designed for this.

 

[QuickHitRecord]

Here is a neat little trick for using lenses that have rotating filter threads with height-adjustable matte boxes:

1. If you don't already know the inner diameter of the opening at the rear of your matte box, measure it with calipers.
2. Go on eBay and order a lens hood with an outer diameter that matches your measurement (I got mine from a seller called mkstudio-us; they have a chart of all of their lens hoods, complete with inner and outer diameters as part of each listing). This will attach directly to your matte box.
3. Now measure the outer diameter of the frontal lens body. If it is significantly smaller than the inner diameter of the lens hood that you just purchased, then you're going to get a light leak. To correct this, purchase a step up ring to screw onto the filter ring of the lens that also has an outer diameter of less than the inner diameter of the lens hood. You don't want them to touch, just to be as snug as possible.
4. If you really want to protect yourself from a light leak, or you need some room for a focus gear, then you can also purchase an additional lens hood to screw into the step up ring (but make sure that its outer diameter is less than the inner diameter of the first lens hood).
5. Mount your matte box to your rods, clamping your lens hood, and screw your step up ring to your lens (add your second lens hood if you bought one). Push them together, so that the lens hood is resting on the step up ring. Adjust the height of your matte box until your step up ring has enough clearance to turn without making contact with the lens hood.

The inside of these lens hoods is ridged and matte black to absorb light, so it seems to work pretty well. I like this solution a lot more than attempting to use conventional "nuns knickers" because there is no friction, and it's better than sticking to round filters because any slight marking or dirt on the glass can become distracting when it is rotated.