I put them under a heavy weight (well, a mug press on its side!) left for ages until totally cooled but still got small warp. Tried everything over the years, suspend them up with weight in middle, re-heat etc. Very rarely get one flat which is why I choose not to do many. There has to be a better solution, Magic Touch do the medite ones, might be worth a look. Printed a different way though....and not much cheaper.
I use Unisub coasters but also the wooden ones from BMS etc. I wonder if wooden (not unisub) placemats are available?