Box squats are an aid in developing your squating technique. It's mainly useful for competitive powerlifters. I'm not saying it's not useful for you but you should be looking at lifts based on weightlifting, not powerlifting. Nevermind the names of these sports, but weightlifting will generate power more while powerlifting is more strength. Obviously there is a great deal of crossover effect but if you want to focus on power you need weightlifting techniques. Already mentioned are cleans, snatches and jump squats. Here are some more:
http://www.exrx.net/Lists/PowerExercises.html
I find it best to do power exercises early in the workout and strength exercises after. Power exercises require you to be fresh as the technique needs to be right on and power disapates faster than strength.
The normal rule of thumb for jump sqauts is that the total system weight (body + barbell) should be about 40% of 1RM. Therefore if you are not squatting well more than bodyweight, just do bodyweight squat jumps and don't worry about extra resistance. For example, an empty 45# bar would be suitable for a 200# person capable of squatting 400#.