The tank divider I'm currently using is a DIY with a plastic needlepoint board and those page protector clamps. I need it to keep my rescue crawdad Skitters away from my new betta and cory cat. The problem is that I've come upstairs to find Skitters on their side once and Slipstream (the betta) on Skitters' side.
The divider is wedged in well, a bit bigger than the actual width of the tank, and the bottom is covered with several inches of gravel. The top is also about an inch or so above the waterline.