Dividers the Pennplax are the best bets. Though slide binders and tapestry canvas (plastic) are meant to be good too. Will be trying this method myself out over the weekend!
i'm also dividing my tank into 4 sections, with the help of Craft Grids and Paper Binders. I ordered off Ebay, so waiting for them to come yet!
Cant wait!
For that size tank, you can divide into 2 perfectly, and if your worried about the filter, just turn the outlet towards the glass