Bettas are jumpers. There are many stories of Betta owners with divided tanks coming home to find that one of their Bettas jumped the divider and attacked or even killed the other fish. If there’s too much of a gap above your divider or if you have a particularly determined Betta, this can be a real risk.
As a safety precaution, you can use a Betta tank divider or craft mesh to make a jump guard for your tank. This prevents Bettas from getting to the other side of the divider. If they try to jump the fence, they’ll just hit the jump guard.
To make a DIY jump guard, cut a piece of craft mesh or tank divider material into a strip that’s about 6 inches wide. Place it over your tank divider and fasten it to the divider using either fishing line or zip ties.