I built mine with wood floor. It's on the small size 10'x12'.

It's on 4x4 PT skids plus 2x6 floor joists and 3/4 plywood floor. I use left over steel roofing under the evaporator including about 3' extending beyond the door to catch any random embers falling out.

The benefit is I could move the shack in the future if I need to.