I just recently divided a 40B into two cubes. I personally would not want anything smaller than the 20g cubes and think it is just about perfect size. I got a piece of glass cut and polished. The bottom corners were rounder slightly so they didnt hit the existing silicon filet at the botton seam. I had the glass cut a 16th" short so there is a 1/32" space between the panes of glass and the divider so I can put a silicon fillet between it. here is a pic.