Gah why is Rollitup so damn slow of late... couldn't quite load up big versions of your pics just yet, but from the little ones they're looking really good! they're about the same height as when I used to flip, averaged about 2-3oz a plant.
It wont be impossible to train in that box just difficult to get to the back once they start filling it up
You could keep a female mother plant in the pc box, take a clone or 2 every 3 weeks, once they root veg a bit, top, rest a week and put them in flower at 4 week intervals, that way you can harvest 1 plant every 4 weeks with about 2 plants in the flower box at a time. You'll have to calculate your times better than that (I'm just estimating lol) but so long as you have 1 rooted, topped, recovered clone every 4 weeks you should have enough space for a good perpetual of about 2-4 oz every 4 weeks or so (depending how well you dial it down as grows progress!
The reason I say do it that way is your limited space (before anyone jumps in and says sea of green would be better lol) 2 medium- large plants in your area will be more efficient than say 5-8 rooted clones straight to flower, you'd get more regular harvests but I reckon it'll be a little less in weight as you need established plants to yield the best, that's my ethos anyway
As for your 2x2x2 cooling shouldn't be a problem if you want a 250w HPS so long as you can fit a cool tube in it, just run the ducting outside and mount the fan on the outside, unless you have space on the inside but I don't think you will, cooltube is about 45+cm long fan takes up a good 25-30cm. ANYWAY the moral of the story is XD so long as you have a good exhaust and a cool tube you'll be ace!
Sorry I always seem to write a stupid amount for such a simple reply lol
Stay Frosty!