I was thinking it seems a little small too. We were thinking about using an old speaker box.
You just need a smaller pot. I have seen people grow out soda bottles. I like the setup. I would say make it work since you have spent the time building it just get yourself a long horizontal pot or tray
Personally, I think you should re-think everything.
It is a ton of trouble for such a small yield. Why not just use a closet and get some real lights and then you will actually produce a product worth putting the time and effort and money? There are plenty of ways to make a grow box big enough and good enough to produce for personal smoke and still be stealthy about it. Like someone pointed out above, a hollowed out dresser or chest.
My buddy has a box in the middle of his art studio and has it painted and looks like something he stores materials in, its actually growing MJ.
heres mine and its in a box about as big as a pc. 12/12 from seed tied down with rubber bands and topped.
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If you keep it simple, keep the number of variables low, and pay attention to what your plant tells you...it might end up surprising you, too.
You have like 4-6 inches of room think about realistically plants stretch more than that just during the first week of flowering...Besides that you will also have concerns with heat issues.