That "wet paint" glossiness look is from the plexi coming into actual contact with the CP. If you can press hard enough on an area that doesn't have that look, you'll probably see what I'm talking about. If that actually ISN'T it, I don't have any ideas yet on how to fix it.
If that IS it, I have a solution that may or may not work, depending on the thickness of your CP. Put a spacer between the CP and the plexi. Then when you tighten it down, it should give enough room so that it isn't pressed/held so tight that it causes that effect.
What to use for a spacer.....the first thing that comes to mind is a washer, but I know it might be hard to find one that doesn't peek out from under the trim ring. The next thing that comes to mind is a rubber O-ring. Check the plumbing section of your local hardware store. The third thing that comes to mind is a gasket, search your local automotive parts store. The last idea will require you to create one yourself. I'd bet PVC would work, but I'm not sure 1)it won't peek out, if you find the right size, and 2)you'll need access to a power miter saw or table saw to slice it thin enough to be unobtrusive and straight enough to not cock up your plexi. Do NOT try to cut it with a hand saw/miter box. You'll more than likely break it halfway through the cut if it's thin enough to serve your purposes.
Sweet idea on the gutter mesh! Necessity IS the mother of invention! How wide are those pieces of mesh? I thought they were 4-6-ish inches wide.
To further my flashing light idea, I would want the light to flash (catches people's attention better that way, I think) if no music were playing, solid-on when music is selected (or after queing up a playlist).