Your 600 EX should have a slave mode so that it can be triggered with another flash.
As far as soft boxes, you can get away without a strip box by covering the box with black cloth if necessary. It depends how you need to control your light.
In regard to a softbox recommendation, it will really depend on your budget. I recommend you check out BHPhotovideo to get an idea of what is out there, as well as pricing.
Depending upon how involved you become, you will learn that light control can be complex and fun at the same time.
One more thing, be sure that whatever softbox you purchase has a grid that can be purchased as well to reduce light spill.