Craft knitting sheets
Report Cover binding sleeve
I use the clear knitting sheets and put the binding sleeves on all 4 sides just didn't have any pics on phone and I'm on vacation so found one online that looks similar. Total cost was about $6 for 3 dividers which I put in 10 gallon to seperate bettas
I used corrugated plastic. I picked up a giant sheet for $5 at Home Depot. I cut it to size and hold it in place with suction cups. That way the fish can't even see one another, and it cuts nicely with regular scissors. I did put a little inset of craft mesh along the bottom and top to promote water flow.