Hi
I like your approach - Im also renting, and have downgraded my massively obvious garden to a nice little cab. Its only 1[w] x 0.5[d] by 90cm[h], and it handled a 250w cfl without heat probs using only one 8cm pc fan extracting. Im planning to put a 250w mh/ [eventually] hps in there soon, and Im sure it will handle the heat once I up upgrade the fan/s. Lighttraps are essential for stealth, and quiet ventilation is important 2. I found an excellent way to cool the thing - run intake venting from the fireplace/chimney. I used a smaller cab and some clothes to disguise the vent snake, and that air is even cooler than the air outside, all the time. works like a charm.
A cooltube would work well too, but it means constructing more vent holes [unless budbox has enough already?], another extraction fan, and lighttraps to cover the extra vents, which are a pain in the arse when youre trying to save space and noise. Normal extractors? Are you talking about RVKs? that would do fine, im managing with sneaky pc fans [25 db, quieter than my actual pc, which runs like a doomsday machine]. You know LST? do that, keep em bushy, and you'd be ok. I can only manage 40 [under bulb]-55cm [furthest away from middle] of growing space, but that suits me fine for quick harvests, they need training obviously, but it means I finally develop an actual canopy. The question for me is whether I can get a 400w in there...
Its always worth it to go stealthy for peace of mind. Before, when anyone walking into the room would have meant game over, I was worrying about it all the time, especially when I was out of the house, or near harvest, or just smoking... it just wasnt worth that stress every time someone rings the doorbell [could it be the landlords? quite possibly...]. Now everything is chilled, and I dont feel like so much of a criminal
gd luck