By placing horizontal iron beams on the floor, one can then place a hearth or cooking fire on top of them, or place stone flooring pieces on the iron bars then one's choice of fire source on top. I spend a lot of time repairing stone keeps I come across in the wilds, and I make 2nd level hearths in them. I usually place them where the vertical archer peepholes are, and usually place down 2 of the largest stone construction pieces laying horizontally flat. This requires the "advanced building mode" of a certain mod, which allows one to rotate any building piece on the x, y, or z, axis.