Just set this up to see how it goes...
* 10g standard glass tank
* Substrate is native Central Texas dirt from the yard, capped with Flourite Black
* Lighting is two SunLite 13W T5 Daylights in GE Slimline under-cabinet fixtures
The new part for me is the filter:
* 4" cube matten-filter with Airlifter from Swiss Tropicals
The rest of the divider is made of:
* 2" matten-filter foam also from Swiss Tropicals
* I used some cut piece to block the space at the top of the cube filter so the bettas can't get across.
* The foam sits on the glass bottom and touches the lid, so there's no way past it above or below.
Yes, the 2" divider is pretty thick, but it holds itself up and doesn't look too bad. 1.5" or 1.0" could have been a better choice, but I think some support would have been needed.
The original idea was that water passing through the divider would add to the biofiltering capacity, but since I put a T on the airlifter output (to provide water movement on both sides) there's not much flow going through the divider. I think it's stagnant in there.
We'll see how it goes and what adjustments to make. If the foam up against the glass gets grungy over time, I may add some trim on the outside to hide it from view.