As a little girl I remember my mother sometimes ironing on a doubled up towel over a table. The fact that steam and heat can really damage wood probably never crossed her mind since ironing boards were once made of wood. As a little girl, she would have ironed on a board like this, using a snazzy "Steam-O-Matic" iron like this one. Seems like everything was an O-Matic back then.
You might wonder, why cotton? Because it can easily handle any amount of heat and moisture from your iron without damage. Polyester batting will melt in high heat situations, it won't absorb moisture, and besides, they usually have too much loft. Commercial pads can be made of polyester, but they will have the addition of a heat retardant, like fabrics made for kids sleepwear. They still don't absorb moisture well.
Now that you have your pad fixed up and ready to lay out, how big should it be? Because the edges get a good amount of wear, cut it long enough that it will easily cover the sides of your board. Just trim it right on the board to fit. Together, my last two pads cover well enough. We like to use the end a lot, so my end is much longer than it needs to be. It goes well under, and as you can see I do not need and cannot use those end pockets. Yours does not need to be this long.
Here is the link for how to finish the job right with a cover that is custom sized to your board: