It's beginning to look like a fine mesh, fitted inside the lip. Hopefully it won't have too much surface tension to prevent low rainfall from getting through it. I might even slightly concave the surface of the mesh so that there is some capacity to cope with heavy rainfall. Looks like some experimenting over the next couple of days.
If that works, i.e. it stops the needles getting through and it copes with heavy rainfall and doesn't hold light rain (surface tension), the addition of a blower fan to clear it of needles and leaves should make it all a goer. The roof is some 3 stories of the ground, so a long way to fall
and getting to the sensor isn't that easy either, even though I can reach it from the 2nd story. So I really do want this "fix" to work and make the rain sensor maintenance free.