Varnished plywood*.
*I used water based polyurethane to avoid the smell / fumes, but it really sucks as a coating, for several reasons. Wouldn't recommend.
Not at all ideal, but I don't care about charring it. It's cheap and, ah, "distressed" as they say.
I've somewhat lusted over soapstone. Lightly greased or waxed or varnished. Probably glued to a base like plywood so it's easy to move. ESD might not be the best, but that can be dealt with by additives I think (a very slightly conductive grease, perhaps something with an ionic detergent in it?). (No, no worries about asbestos. The coating keeps dust down.)
The same can even be used for everything up to brazing steel! You'd use a thicker block and no grease or additive, but it wouldn't be very visually appealing after use, so it seems kind of an expensive/extravagant waste there.
Or less rustically, anything with a lot of polyimide or teflon or silicone (in a plastic, rubber or resin form -- take your pick) would serve excellently. Again, give or take ESD, and surface treatment.
(By the way, no, it's not good enough to simply, say, lay down a bunch of Kapton(R) tape on a wood surface -- that stuff more than handles the temperature where wood pyrolyzes, so it would just char and bubble up under the tape.)
Tim