trouble with dual veg and flower

I started by plants from old good chronic beginning of June. Only 1 grew and it is getting large. I picked up Afghani seed from West Coast and 7 out of 10 germinated, only 4 came thru the dirt. I have to hang onto all of them to see sexes. I have them all in 1 closet under a 400hps 24/7. I will have to take the first plant out soon to put it into flowering. Any suggestions on how to do this. Could it be put in a black box with a lid for 12 hrs to flower at night and taken out of the box under the light for 12 hours?

I'd keep the closet for flowering and continue vegging under some shop lights elsewhere, because you can stick a shelf with a couple 4' fixtures on 24/0 over it ANYWHERE with no fear of photoperiod problems.