well i've been working at it since about 9:30pm tonight (~1.5 hours!) and im still not happy with any idea i've tried!
my first attempt had the filter on the far right with three dividers creating 3 partitions besides the one where the filter would be on. because of the small size of the holes in the dividers, the filter wasn't doing anything and it looked as if the water would just be cycling in the partition of the filter!
so my latest idea (still working on it as we speak) is to put betta | filter | betta. so the filter is in the middle and the dividers are on the side. i'm still unsure as to the performance of the filter in such a situation
i've tried making the holes bigger but i end up with an ugly "volcano" shaped hole afterwards (it bulges out to one side).
any suggestions from people who've done this? thanks!

i'll post back if i come up with another idea or if i finish this one! back to work!