If it’s that dark, I wouldn’t worry about getting a tent later on. I have my tent wide open and in a room that has plenty of light leaks. I’ve never had a problem doing this. No Hermies…period.
Is the ceiling in your room made of a material you can put staples or thumb tacks into? If so, you might consider buying a roll of reflective mylar and hanging it from the ceiling, making a reflective surface around the sides of your plants.
Also:
The strains you select might effect how big of a space you make, but with a 600 watt lamp, I’d suggest making it 4 feet by 4 feet. Of course, since you would be making your own grow space with the mylar, you could even make it circular if you want.