I have used crafting mesh and have seen fluorescent light crate material however my latest favourite divider is Poret foam cut from a bulk sheet. I got mine from AngelFins - expensive but really versatile. The Poret foam also becomes part of the tank bio-media.
I use eggcrate (AKA: plastic light diffuser; sold at Home Depot etc.). It's easy to work with, totally customizable, and if you have different sized fish that you are trying to separate (like when breeding large, aggressive cichlids) you can easily make holes within the divider so that the smaller fish can move through it easily, but the larger fish is unable to get by.
If you size it just right it holds itself in place - no need for suction cups or anything like that.
The only drawbacks are that it looks god awful, and being white, it is super bright - which means it grows a lot of algae on it. Which I don't mind as it camouflages it a bit.