I'm more "salt" than "fresh" but I *really* like Tahitian Moon Sand and supposedly you can use it in both (I've only seen it in salt - but then again, I don't frequent freshwater boards).

I would assume you could cut/drill plexi glass to create false walls (walls that would block the fish but allow water to flow through all chambers).

I've always wondered about multiple males being housed in containers where they can see each other.... does it stress them out? (ie Is it healthy?) I don't know the answer to that but it might be something to consider.

My betta is housed in a 3gal acrylic tank with lights & pump/filter but no heater. (Though it's only on during the day, the light warms the water and I have about a 2 degree flux between day/night temps.)