My .02...
Stealth fans: As a rule, the smaller the fan for a given cfm, the louder. Small, high-cfm fans scream. Big, low cfm fans are quiet. So for stealth, go for the biggest fan with the lowest cfm. You could also place the fan in a light baffle/muffler to reduce its decibels. Even pointing it in the right direction can help.
Digikey.com in Minnesota is a leading supplier of electronics parts and a really good company to do business with. They have a HUGE online catalog and friendly people who will actually answer questions. They stock lots of equipment cooling fans, which is really what you're looking for, and they list CFM and more importantly decibels for each unit. They will probably cost a little more, but that's the price of quality and stealth.
Back pressure from a scrubber is a complicating factor. Maybe contact your scrubber supplier for assistance?
Consider a low-odor strain and venting out a window or whatever so you can just skip the scrub.
Don't forget that you can do a lot of cooling passively by convection: big intake holes at bottom, big exhaust holes at the top, hot air rises, you get the idea. Then maybe just a small fan to assist. Remember that when your plant bushes up its going to block air flow.
You can research "light-proof baffles" or related on Google to find systems that people have created that let in lots of air put don't let light escape. Again, it requires a lot more work and fussing, but such is the price of stealth.
Finally, I can tell you from personal experience that
you will be disappointed by your results with four 23-watt CFLs in a six square foot growing area. Each plant will need at least twice that to give good yields. I would suggest researching a 300-watt or better LED unit and mount it such that it's exhaust fan exhausts not into the box but outside the box. (300 watts would be underpowered for 6 square feet by the way--that's really more a 4 square foot device from what I have read).
Cannabis needs BRIGHT light to thrive. If you don't need sunglasses to look at your grow, it's not bright enough.
Come to think of it, though, with your height constraints, you won't be able to get an LED unit far enough away, so instead a better bet would be four-foot fluorescent tubes in each corner.
Consider dwarf strains. I have had good luck with CropKing's Dwarf Low Flyer, but even it outgrew my little fireplace grow box. Alternatively, as you may know, you can just start you plant on 12/12 or switch to flowering very early (@ 1 or 2 weeks) to keep size down. It's great to watch a healthy plant explode in size at the flowering stretch, but when it grows out of its box, that's a problem.
Stealth is tough but kind of a fun challenge. You can plan and plan, but as I think you have seen, you kind of have to make best guesses and refine based on experiences. You might want to google "stealth grow box" or have a look at amazon.com to see what others have done so you're no reinventing the wheel.
Good luck, have fun, and please post your results.
When this little Dwarf Low Flyer went into its flowering stretch, it outgrew it's grow area.
