Have you thought about white glossing it? When i i made shield it came as plain white but it was still reading high, so i white glossed it and that made a difference of about -1C
Once upon a long time ago, when I was helping to run a geophysical observatory in an inhospitable climate, we used to repel pesky radiation from non-scientific things with aluminium (sorry, Niko, aluminum) paint. But I did notice that, e.g., guard plates on solarimeters and other scientific kit were gloss white.
And I think I have seen a posting somewhere by the author of this thread re polishing the white radiation screen on his Davis?
That’s OK mate, I’m bi-lingual :lol:
I do think that alu tape is the best solution to the translucency of the plastic material, but you could have a good point about adding a white coating. I found this interesting chart from solartoday.org
As well as the reflectance (which shiny metal does quite well on), we also need to look at the emissivity (which shiny metal does very poorly on). The emissivity is why when you need to keep things cool in the sun you generally use a white surface rather than a shiny metal one - it has good reflectance and emissivity properties.
Example: leave a shiny spanner (wrench for those less bi-lingual than Niko) in the sun for an hour - then pick it up! Same reason they now no longer orientate children’s slides with a metal surface in a southerly direction in play parks/schools.
Sounds good to me 
Actually, we do have “spanners” 
It’s the term used for the type of wrench that has fixed or adjustable pins to fit into holes in the object being wrenched, or like the tool you would use to unscrew the fixing ring for a camera lens.
Reflectance, emissivity. . . Now we’re getting to it! I told you you weren’t alone, Devil.
Ah! C-spanners.
And…
Since they appear to be thin white saucers maybe doubling them up would help now:
White one
White one with alu tape coating
Space
White one
White one with alu tape coating
Space
etc.
Until our recent “heatwave” I genuinely had not known that we now paint rail lines white. . . being welded these days, they don’t seem to have expansion joints.
I’ve never heard of that, but I have heard of Rail Stressing.
Here’s a link to Network Rail in UK.