I am having some dividers made for a 20 long rimmed aquarium. They will be opaque and made of .25" thick acrylic. They will have .25" holes spaced evenly apart. This allows most Nano fish to utilize the entire tank while keeping the Betta secure in their sections. I have seen Neons, Pygym, Habrosus and Hastatus, Dario Dario, etc., swim in and out.
These dividers are so heavy I don't even use suction cups. However, you can anchor under the substrate (see photo). Mine fit so snuggly in the tanks I bought at PetSmart that they do not need side suction cups so it's a cleaner look. You can use a 3" wide strip of stainless mesh or fabric mesh across the top if you wish. But with the canopy fitting right on the slip of the inside rim I've not found it necessary.
Using Krylon Fusion, these dividers can be spray painted any color. I have done so with no issues. I let them dry for three days before using in the tank.
Cost will be $15.00 per divider + shipping. Less sturdy dividers are selling for $20+ on eBay and elsewhere. I will also be getting the same dividers except for 10 gallon rimmed tanks. I'm waiting for pricing.
If you might be interested please send a PM. Still waiting for the ETA.
BTW, the bottom photo is how I like to divide a 20 long: Sections on end are 7" wide and center is around 16". I swap the Betta from one section to another.