The foam works, like Tim pointed out. This keeps most of the reflected light from coming back in around the opening where the screws are. I used mini-cell, gray, most common in whitewater kayak walls. I cut out a ring of the foam based on the diameter of the Universal adapter, then made an inner cut out for the lens shade. This works, fine the mini cell has a good grip and holds the X-Pro on well. Then mounting the Glass sheet from Schneider is the final step. I have yet to find a really quick way to get the mounting of the glass, but Velcro sounds like a good idea. You will still need to cover the top of the rig as the glass sheet pulls in reflections from the top, I used a dark cotton cloth for that. Over time Mini cell will harden so it will need to be replaced over time.
Tim, did you use the Velcro self adhesive style? It gets a good enough grip I guess.
I also found out that most of the time, when I had the rig 100% polarized, the sheet wasn't lined up square, most times it was lined up at an angle from 90 degrees.
In shooting over the weekend, I never had a great feeling that the sheet wasn't going to fall off the way I was securing it, so I need to move to the Velcro.