I think you mean horizontal or top,

wait you are in flowering correct?
you need to have complete darkness during lights out. So yes you need come up with a top and sides that no light can get thru.
Jerry Garcia 2007 Reviewed by Jerry Garcia 2007 on . closet set up possible without box? please evaluate I am trying to revive a plant that had been left, cared for only by nature for the last few months, outside. The plant is a thin and straggly 8-10 in tall with 2 small, 3 fingered leaves and a few small purple flowers/preflowers emerging. It is currently in a 10 inch diameter pot in my closet, receiving 12hrs of light (currently at 5700 lumens, from 4 fluorescents) and 12 hrs of darkness a day, as that is what it had been getting outside. I do not have any kind of special box as Rating: 5