how big of a deal is lightproof?

you may need more ventilation (exhaust?)
you can go 18/6 anytime you want, if at all, why not start that way?
youll have to drop to 12/12 eventually, the box should have absolutely no light entering during dark periods
are they gonna be in that box throughout its entire growth?

 

Not to bust your nuts but.. I don't know how your going to grow with that box... I have one plant that I have been growing just for shits and giggles out of a bag of swag and its already taller than that and its 4 weeks old.

 

ive seen some crazy things pulled off and i wouldnt put this past you, your best bet IMO is to 12/12 immediately while LST

edit: you may have a prob with the plants touching the lights if theyre too hot

 

Lightproofing is a two-way consideration. Inside the grow space you have to be concerned about lightproofing so that the dark cycle is not disrupted/corrupted/etc. For a 24/0 op I agree that lightproofing for the plants does not matter. During 18/6 it does matter.

But there also is the lightproofing consideration for stealth outside the grow space. Nothing like a light as bright as the sun leaking from under a closet door or from a tool shed or cabinet to arouse a little suspicion. In that respect being fully lightproofed at all times is best.

 

lol, reminds me of a movie where aliens land in a barn (or something like that) and the light of god shines thru the cracks