when steelwork is enclosed for fire protection, it's done in two (at least) layers of ordinary plasterboard, nailed to a wooden internal frame (but I suppose metal lathing would do) and skimmed to fill and cover joints with ordinary gypsum plaster (not filler). You could use plaster inside your enclosure too, perhaps.
gypsum plaster sets by reacting with water, and, when heated, it releases that water as steam, which absorbs heat. It doesn't burn.
gypsum plaster sets by reacting with water, and, when heated, it releases that water as steam, which absorbs heat. It doesn't burn.
