I've haven't yet fixed up a map board for my bike, but I just thought about using foam board (really cheap from Wal-Mart). You can cut it up to whatever size you want. Then cut your maps into sections, and laminate the sections.

I do all my laminating at our local Country Office of Education, which is really cheap. I can laminate a city map for about one dollar, and laminate is thinner than Kinkos, so I can fold the sections easily, or roll them up.

You could also punch a hole in the corners, and put the map sections on a ring .... I still need to try something while I'm actually riding my bicycle. But first, I need to get a bigger hanadlebar bag, and figure out how to attach maps to the bag .....