Have any of use used the corrugated plastic sheets and made your own? I did this a few years ago and I think it saved me a lot. I don't know what the comic boards cost now, but you could compare. Here is a home depot link
http://www.homedepot.com/p/Unbranded...6-15/202038094 it comes out to $4.60 a sheet...you can make them any size you need.
It is a little hard to cut in one of the directions...I used an old rotary blade and it was OK...but if you have arthritis in your hands it might be too hard.