Changing grow space for dark period

My DIY grow box is not light proof due to heat problems, does anybody know if moving my plant to a smaller dark grow box at the start of the plants dark period and moving it back into the bigger non-light proof box for light period every 12 hours will cause the plant stress? And if so how bad of an impact it will have ?

well, if you move it carefully and you place it under light in the same direction, and provided that you never miss on moving her in time, then I guess that you will be fine. I will never advise such a thing though, to grow anything decent you will need ventilation, not just for the heat but for the plant to breathe. don't do this manually, get your box vented adequately, and use a timer.

My biggest concern is that dark room is a lot smaller than other room , plant leaf will be within about 2-3 inches of sides of box .
Will the plant be able to sense being this close too the side and be stressed from it ? Can't afford ventilation a.t.m but will be looking into getting some , currently using small fan inside box with door open , bigger fan inside room and door open and window slightly open (window covered for light).

dude the plant will not sense being so close to the wall, place that fan you already have on a hole in ceiling of your box and use it as exhaust, exhaust fan is much more important than oscillating fan for temp control.