Hardiboard or equivalent in other brands in comes in 1/4", 4'x8' sheets. I use it on my table to keep from burning the plywood below and it's better than a metal top because your slag won't stick. It works!
I believe the 1/2" thick is only in 3'x5' sheets. It will show burn marks if a torch if left in place for too long but will not burn through unless you are careless.
A couple of sheets will cover a 8x8 area.........set your project on top and you are good to go.