Finally, I have them installed. Here's a pic of one.
The "X" shaped backing is coroplast. Glued to the perforated aluminum with 100% silicone glue. Unfortunately the discs aren't perfectly flat so they bow away from the wheel rim a bit on two or three wheels. Not too bad though, certainly nowhere near the maximum width of the tire cross section.
All disc attachment screws have outer washers, but two of them shown here have oversize washers due to one of the holes in aluminum became much too large. It needed an oversize washer, and to keep the wheel balanced I added a matching washer 180º opposite.
Screw type: I estimate that pan-head phillips screws would be most more aerodynamic than hex heads. However phillips screws would require pushing in on the screwdriver which could potentially pop the epoxy'd nut off the inside of the lug nut. So I went with hex head screws (#8x32). That way there's no pressure on the screw head, just the turning action.
All mounting hardware is stainless except for the screws. Could not find them in stainless. I expect to find that before too long and likely will replace them with stainless.
