Our guild had a large wooden board that fit over an ironing board and it was great for ironing the big things. They would bring it when we went on our retreats. The nice thing about it was it had the extra strips of wood on the under side of the board to keep it steady AND it folded in half, with carrying straps, for easy transport. The handles were nice because the solid board does get heavy.
For me, I just bought a piece of particle board that was 4'x2', covered the top with heavy duty aluminum foil, a layer of cotton batting, a piece of heavy duty decorator fabric and stapled it all down. I put it on my bed when I need to iron and sit on an office chair (to keep my back from hurting). I store the board under my bed when not in use. The only thing I don't like about it is the smell of the particle board when heated. If I had had the money I would have used ply wood, but since I am on a budget, the particle board was only $5.00 (USD).