the octagon without and central growth might push it upwards of 15-16" L x W?
so tight squeeze for 2 rows unless you shape it longer than wide. Definitely want 2x4 on the colas if you want more plants.
that's a cola per 6" going across, which is just about perfect, especially considering you are mainling. Maybe only 3 rows deep though. that'd be 12 colas and 4" between. So 4x6" space for each, can LST for best fit within that to even it out dimensions if you wanted.
six plants given 8 colas each.