i'd also recommend (depending on the reasons for the divider) aquatic mesh.
a lot of people use these for things like moss walls and the application for a tank divider is fairly similar.
you can buy mesh from most LFS
i believe, or even hardware stores (bunnings if you're in australia, or home depot in the US i believe?). just don't buy any with metal.
simply cut to size, leaving approx 1-2 inches of additional space on either side, and place suction cups through the mesh down both sides. place in tank, stick the suction cups to the back and front and voila. this gives plenty of water flow if required, and doubles as good grounding for plants or moss you might use with it (if you do). if you have substrate, place an inch in it so nothing can go underneath also.