I really don't get the box thing, my advice would be to use a prong collar.
The point of the "box thing" is to teach rear-end awareness and, later on, help the dog find the desired position from a variety of angles and approaches. Depending on the size and orientation of platforms that you use, it can also be extremely helpful in teaching correct position changes (Sit to Stand to Down and all combinations of those three basic positions) and building the dog's core strength (dogs with less core strength tend to Sit sloppily, can't hold the position as long, and may throw their front legs forward when going into a Down, instead of lowering themselves in a fixed position).
Personally I really dislike using a collar of any sort, let alone a prong, to teach heelwork. It can
work, and it can work beautifully, but it's not my preferred path. I've seen dogs who did just fine with it, but a lot of other dogs trained the same way exhibited a lot of stress and avoidance behaviors, and it's just not something I want to see in my own dogs ever. It's not that difficult to teach competition heeling via shaping, platform training, and targeting; I don't see a need to use compulsion to get there.
ARCHMX TDCH Pongu the Insane, CD-C, RE, RL1X6, RL2X5, RL3X2 (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, ITD (Akita mix, b. Jan 2011)