DIY some REALLY skinny, REALLY deep pots and u could do like 6 or 8 maybe, pruning your bottom branches off mostly to concentrate on the colas...not that such a narrow space wouldn't take care of that for u anyhow. how wide is a 2L coke bottle? you cuold take two of them and piece them together to augment the pot vertically. i think i'm going to be doing something of this sort, keeping 6 plants in a single 18gal tub in potted soil. i have about 7 ft. to work with and plan on flowering with a 1000w HPS basically supported as high as i can get it (i'm in a closet, not the gay kinda).
the soda bottle would not be left clear.