My friend's DH made one for my friend and me. We used the half sheet of plywood that was 1/2 inch thick, we covered it with 2 layers of cotton batting, then 2 layers of cotton fabric. We pulled the fabric over the edge and stapled it to the plywood. He then put a one inch finishing piece over the fabric and nailed it in place. Since we were going to be setting them on to an ironing board we drew line around our ironing board and nailed 1 x 1 boards on the lines so the board would set onto the ironing board and not tip.
So far it's been over 10 years and the fabric is still looking great. If it does need to be changed I will more than likely just make a cover with elastic and ties like my ironing board cover.