First bit of advice will be to insulate your attic space as much as possible. I also grow in an attic in the northeast... draft control is HUGE, plastic window sealers are a start, whatever you can do to retain heat at night. A thought would be to run 2 lights on opposite schedules and take advantage of the ballast/bulb heat- build a pair of boxes and simply exhaust each into the attic as a whole, to make temps more even without a space heater. If you want to do one big box or room, you'll want a small oil-filled electric radiator on a thermostat set to kick on at <65'F. You'll be running your lights at night to take advantage of lower electric costs and lower night temps.
Just some thoughts to get you started.
Basements are better, lol