I have a 2.5 gallon "betta bow". Two main problems with the divider in this tank are the bettas can see each other really well and there is a slight gap along the sides because of the bow in the tank. I've read several reviews about bettas being able to get around the divider. So I decided to tweek the divider a bit to make "visiting" more difficult.
I took some plastic mesh and some report spines and cut it to fit the divider that came with the tank. I placed the mesh over the divider and then slid on the spines. You won't be able to slide the divider down in to the grooves from the top because of the spines so you have to go from the bottom up. This took a little bending but with a little effort the divider popped right in to the grooves. A warning though, your tank may get scratched while trying to get the divider in place. Once in place it is secure and the bettas can no longer squeeze around the divider and the mesh blocks their view much better but water can sill circulate through the tank.