Every once in a while I like to check the temp of my heat sink, and I do this by actually touching it near the COB while the light is running (never been more than warm so far). Obviously I don't want to touch the COB, so I need to be able to see what I'm doing. I use these, about $9 from Home Depot. They are good for any time you need to work on the light while its on.
@FranJan - I am thinking about making some home made reflectors from individual muffin tins. How did you attach the one in your picture to your heat sink?