well, i have recently been trying to find a tank divider to go into my 6 gal, so i can have 2 bettas in there. This has prooved a little tricky, as i cannot find the specific size anywhere. Then the other day i saw one of my old Under Gravel Filters that came free with a tank i got and have never used. Then it hit me that that would make a perfect betta tank divider! It has gaps in it to allow water flow, but not big enough for our betta friends to catch site of each other!
I took the measurements of my tank, and then cut the UGF to size, sanding down any sharp edges with a nail file, and rinsing it under a tap to remove any excess plastic
This acts as a perfect divider, at a very cheep cost. Even if you dont have a spare UGF, they are very cheep at lfs's, about £3.99 ($6.50?)
You can also make the gaps a little bit larger with scissors if you want to, so you can weave plants in and out of it, to make it look prettier, i spose thats a personal choice