Never underestimate the power of redneck ingenuity. Lol seriously.
I'm going to try to make a mesh box out of zip ties, the mesh canvas stuff (went and got it at Hobby Lobby), and possily some eggcrate (if it needs more support).
This will be my third attempt at making the boxes - on each of the past two mornings I've come downstairs to find three Banggais in a single container...grr
I tried looking at their vents and did see what that article was talking about. I'm growing a batch of BBS to feed them so that their bellies will be super-ful will stick out. That way the vents will be more visible. So I think I'm going to try to transition from a "test-pair" method of looking for signs of aggression to physically sexing them. Then I'll keep them in pairs for a week prior to sale (just to double-check my vent-peering).
If I sit on the couch and watch them for awhile, they do seem to hang out in groups of two or three. The groups of three are more active, swimming towards each other, away from each other, back together again, etc. My guess is that they're sorting themselves out into possible pairs. I'm going to try to "respect their choice" if possible by herding them into certain boxes. I have to take a pic of this with my wife's phone - they're much more visible in the 20long tank than they were in the rubbermaid box.
EDIT-
So I checked my two adult fish using the "vent" method (discussed above), and they have the predicted number of vents: one for the female and two for the male. The babies are a little more timid than the adults, probably because I've been trying to force them into little boxes all week.
I took a look at the fry to see if I could see any of their vents. The results were ambiguous - I saw a vent or possibly two on some of them, and on some fish I saw none. I'm going to try again once they've gorged themselves on BBS and some Ova. Supposedly it's easier to do when they have full bellies. Not sure why I'm logging so much info; hope it's useful to someone one day. I think the main reason there aren't more people keeping pairs of Banggais in home aquariums is that they're difficult to sex.
EDIT #2 (editing my posts helps keep the number of entries down) - I managed to use the "vent method" to sex two juveniles. I put them into a plastic canvas mesh box (thanks LSU!) I'll come home today and observe them but I feel fairly confident it's a male/female pair.