I use the student display boards you can purchase at Hobby Lobby when they put them on for 1/2 price. They cost a couple of dollars that way, they are 20" x 30" and they are white corregated plastic. You can also buy larger sheets of it. This works very well for my needs.
As you can see this is a work in progress and have only been working on this a few weeks in my spare time, but I'll get there.
PS... My DH cuts them on his band saw for me so each one is "perfect"!