go to lowes and buy some egg crate. it comes in 2 feet by 2 feet sheets. then your gonna cut it in half, and make on piece like 1 square on each side smaller. what i did was put the 2 pieces together so that the egg crate holes are smaller so the fish wont slip through. next zip tie them down. get a thick pvc pipe and fill it up with sand and block of the end. zip tie it to the bottom of the egg crate on both sides and the divider will stay in place. i have a 240 gallon tank and it only cost me $20 its very very simple to make. you see what i mean your gonna have to shave off every side a little so it fits in snug and perfect
good luck