The most creative solution mentioned was to use acrylic sheets that are moderately thick and drill enough holes of some certain size to allow flow. There is a thread floating around about it for dividing tanks.
I think he used an aquarium safe grade of calk that they use in building fish tanks and used that to hard set acrylic sheets (with plenty of holes drilled in them) into his tank.
I suggest no more than 3-4 divisions (2-3 dividers) in a 10 gallon tank. Ideally, I suggest dividing a 10 to have 3 sections, with 25-30% of the tank at the ends for the bettas (keep the distance so they won't see each other and flare), and use the center 40-50% for your filtration, heater, larger bulk of moss/plants and any other fish that can't mix with your bettas (granted the bettas do not see the fish in the center and throw a flaring fit).
If you can find it, use an opaque acrylic, and then you can actually place three bettas in a row divided, without them being able to see their neighbors and start a flaring contest.