Plywood would work. However, I would try to find something that would not absorb moisture, would remain dry and not allow for bugs to make homes in it.
With that being said, this is the perfect time of year to salvage some coroplast from the sides of the road, it only grows there about once every 2 years or so. (The plastic stuff that election signs are typically made of, this stuff is great for all kinds of things around the coop...)
Cut it to size (can be done with a good pair of scissors..) and wha-la... divider... If you let one side get more then 25% less feed or scratch, you will probally have a problem with the coroplast bending, i would take some 2x2's and attach the coroplast to it running horizontal (3 would prob be good depending on the height of the trash can...)
I use 5 kitchen sized trash cans with spring assisted lids to sort my feed and other consumables...
*Edited @2114 07NOV2010 for spelling.