Changed my mind on solid dividers. I had the rather ingenius idea of egg-crate. Solid, safe and very porous but then thought of how to cover up the fact it was egg-crate which would be kind of unattractive. I'm going to grow Java Moss on the dividers and make Java walls.
Eggcrate isn't a good ideas; you would be taking a risk of the bettas fins passing through and the other betta nipping it. It also doesn't obstruct the view very well even with java moss. A risk not worth taking seeing a betta could probably sneek through it.
MermaidMel,
Yes, it is used to actually house the bettas. This way I don't have to have 6 individual 2 gallon tanks scattered all over the place. Plus this way the water is heated and will be hooked up to a filter.