This is it. Welcome to the club, indeed!
Edit to be more useful:
I use 12”x12” mirror tiles. I’ve bought them at IKEA, Home Depot, and Lowes. They’re all similar.
I keep them in place using a sheet of rubbery heat sink / thermal pad between the aluminum bed and the mirror. The pad is tacky enough that nothing slips around.
ZUPAYIPA 400mm x 210mm x 0.5mm Silicone Thermal Pad for CPU GPU Heatsink https://smile.amazon.com/dp/B01N9HGKR1/
I might experiment with something like this. If only it came in black...
https://www.amazon.com/FBApayipa-400mm-Silicone-Thermal-Heatsink/dp/B01N9HGKR1
edit: found one in black