When we made it, my friend and I used the 1/2 plywood 24 x 48 inches (it was what we could lift comfortably), 2 layers of Warm & Natural cotton batting, then used a heavy cotton to cover it. Then we stapled it down and nailed a "finishing board" over the edge and nailed it down to secure the fabric a little better.
We placed the ironing board on top of it, drew around it, attached 1 x 4's so when placed over the ironing board it wouldn't move.