got a problem here my friend; short and bushy? a SOG is top cola's only that yer after not this type of grow yer mentioning. wrong terminology.
what you want is; as you only have a 400w for light is a maximum of 8 plants, right? if you stick to a true indica and flower at 12", should get 3' finish. K2 would be nice. you could fill the bottom of yer box with dirt; creating a FLOWER BED to a height of 9" - 12" allowing the area around the plant sites to be slightly higher. each plant would thus grow in a crater like spot in the bed. like pushing a soup bowl down in a level surface and planting in the middle of it. by raising the outer levels by 4" more would give the roots more expansion space, remember they go sideways as well as down. i've grown this way and it works - just forget the pots completely. an indica would finish flower in 6 - 8 weeks so ya dont have to worry about "root battles". box would have to have drain holes and a 1" layer of lava rock on bottom.