For quilting, it's generally considered best to have a fairly solid surface (not too soft) in order to get crisp seams. Sharon Schamber has several Youtube videos that show how to make a pressing board. You could use the same information for your ironing board. Here's a link:
http://www.youtube.com/watch?v=4LGbXou_u4c
I would not use insul-brite at all. I made my "big board" with one layer of Warm and Natural covered with a layer of coarse cotton fabric (sorry, forgot the name; it's inexpensive at Walmart).