I seem to remember these, did you post them a few years back? Great look, lots of visual interest!
For coasters, cork is quite heat resistant and unless you are placing a red hot frying pan right off the oven top, I doubt you would ever notice. Of course there would be no plastic/glass over the corks 👍
I have made similar but smaller ones without raised sides to use as hot pads. Great gifts. It looks like you did what I have done by cutting the corks in half length wise and saving the side with the labels on them. I cut mine on the band saw. I have even made them as kits. I make the wood part without the corks installed and given them to people to glue the corks in themselves in their own pattern.