Bettas like having their tank heated. A school of 5 White Cloud Mountian Minnows would work in that space. They like having unheated water and don't do much as far as bio-load. It's nice to see you took my advice about the styrafoam "river."
I would love to make things like this for people but I think I would like to get like $40 for what I did here. Does anyone think that sounds reasonable?That is to neat lookin. If I ever decide to go this route I will hollar at you to carve and ship it to me. I don't think I could do that, not even attempt.
I would stay away from bettas in your case because they like HEAT. Like 80+ would be suitable for a betta. Also they don't keep in pairs, males will kill females if she isn't ready to spawn in his territory and if she can't escape he will think that she is refusing to leave his turf and running off maybe ready to spawn females. Good Luck with whatever you do decide to go with.