I've used pressed cork sheet for replacement mats. Cork works VERY well as a material for mats.
You can find 12" squares at most craft supply stores like Michael's or Ben Franklin. What you'd want is 3/16". That's just under 5mm.
Here's how you make one.
Locate the center of the cork and make a hole slightly less than 1/4" in diameter. I used one of those small snap-off blade utility knives but a very fine X-acto Knife blade would work well, and maybe better. A drill is no good.
Then trim the outer edges into a rough circle somewhat larger than the finished size of the mat. I've found that very sharp scissors work best but you can experiment with other methods because your cuts now will be trimmed away later, in the next step.
Now put the mat on your turntable. You'll probably find that the fit is somewhat snug but, once you've got the mat on the spindle, the hole will stretch out and become round if it's not already. If you find it very snug, stop and enlarge the hole slightly. Otherwise, you may pop a chunk out of the cork.
Using a Sharpie, put a mark on the mat at the correct distance from the spindle to define the outer edge of the mat. (In your case, that will be a little less than 5-7/8".)
Now start the turntable. With the platter spinning, carefully put the Sharpie exactly on the mark you made to define the outer circumference. Hold the Sharpie very still while the platter spins, applying minimal pressure. You'll end up with a perfect circle marked on the cork of the exact right size.
Trim the excess in the way that worked out best for you earlier and then sand the edges using something like 120 or 150 grit sandpaper.
John
