Your original solution of some glue and a piece of plywood seems like a fine solution, and honestly overkill imo. I'm open to being told I'm wrong, but the impression I get is that asbestos is completely safe as long as you don't fuck with it. It becomes a problem when you start ripping it out and the particles start floating around in the air. And I've never heard of anyone getting cancer from eating asbestos - just lung cancer from breathing it. If you are worried about it getting in the beer, you should be more worried about breathing it while you are down there.