Just cut some acrylic as close to the inside dimenstions of the tank as possible. I'd add a bunch of holes in a grid too to help with the water circulation. Then scoot the gravel away from the center or wherever you wanted to divide the tank, put the divider in the tank at an angel and then twist/slide it into place. Move the gravel back into place and if it's a snug enough fit it should. I did that with this tank and it seems to hold just fine. This one is a pretty small tank, but it should work just fine.
You can see some pictures of our divided tank if you're interested below:
http://imgur.com/a/VR10r
You can see some pictures of our divided tank if you're interested below:
http://imgur.com/a/VR10r