So....I have a nice slab of 6/4" limestone left over from my hearth installation. (Let me give you some free advice...don't ever try to move a 200 lb. slab of that on your own. Because, well...just trust me on that) I've been thinking that I could top my F400 with a slab of that and get a huge jump in my thermal mass. Then again, maybe all I'd do is melt the top plate. What do you think?