I sealed up the fireplace for the winter. There was a good draft coming in. I used a sheet of plywood 36″ x 44″ with 3 – 36″ x 18″ 3/4″ thich foam insulation boards sandwiched behind to insulate the opening. I used 4 close line 4″ long hooks to anchor the plywood to the frame of the fireplace. Take a look.