I just set up my 2.5 minibow and it came with a divider.
It's completely see thru with huge holes, so I had to improvise.
I had to pick up some of those horrid little betta keeper thingys to transport the wedding boys home in, so I took the 2 little dividers they have in there (one's transparent, one is opaque) and pressed them both up against the see-thru divider that came w/the tank. The one that's opaque they obviously can't see each other thru - but that only covers probably 1/3 of the divider. I put the transparent one across the rest of the bottom part of the big divider. That way they can see each other, but nobody can get at fins.
Then, across the top, I wove java moss in and out of the divider slots.
They can still see each other here and there, but Petey's so interested in looking around his tank that he doesn't really care about Gabriel. Gabriel is completely freaking out when he sees him. I hope it calms down a bit. If not, I'll just have to rip the divider out and put one of them into one of the extra 2.5G tanks I have here.
Wouldn't it be smart of some company to specifically build and sell tanks, like ones up to 20 gallon, which were sold with perfect-fitting betta dividers, divided into as many compartments as you wanted? Hmm....