I feel ya. Venting small spaces is tough. I found a few tricks. #1 keep the veg box out in the open. Putting it in a small closet just heats up the closet.
I just fired up my veg box a few days ago. I was trying to keep it cool with just one fan either blowing in or out. It kept getting too hot in there. I tried it blowing in and blowing out. The temps were Near 80 degrees. I had to fire up my outtake fan that sits on top of the veg box. This keeps temps near room temp. I don't like to run the outtake though because it uses more power and I don't like the little bit of noise it makes. I'll say this though, it really works.
My outtake is just an air purifier cut into the top of my box. It moves good air even on low, plus it does clean the air a bit. It even cuts down on stink if my plants get smelly. Especially if I do some trimming. I bought this for $40 at walmart. I believe it is a Holmes. It draws quite a bit of air. 45 watts I believe.
Another benefit of the air purifier is that it doesn't let light escape. Just like a vortex fan.
Here is the box. I have passive intake, Do I think about making a 6ft tall box? Yep! It would be so much better.
See the air purifier cut into the top and taped with duct tape to make the seal. There are passive holes drilled into the bottom of the box.
The fan in the center behind the pots is no longer there.. Also, Walmart has chorded fans for $10. Way better than computer fans.