Fat Pony is reasonably good with most of the basic "instructions" needed to complete the box exercise and, whilst the fact that I've been teaching him to jump recently may have contributed to his jumping out of the end of the box, the height of the sides of the box would make this unlikely to be a case of mis-understanding.
The most likely explanation for his reaction to the box has to come down to claustrophobia and trust (his trust in me to keep him safe, especially when The New Girl is also hanging around).
So the focus of our training is on building confidence. To do that, I started by making the box open ended, not to walk Fat Pony through, as that would establish his expectation of going through the box, but to stop him feeling trapped and make him more relaxed by letting him see a way out. Relaxed Pony learns better and faster than Panicky Pony!
As I do already have a reasonable level of training in place with Fat Pony and he's not normally prone to panic, my next step was to walk backwards into the box with him following me. Using my body to block him at the end of the box. As the box has high sides, this is not to be advised if the horse is prone to panic and/ or has a habit of running over the top of people! If we were complete beginners, I would have to break this down further and add even more steps in to the training. At the end of the box, I asked Fat Pony to wait for 5 seconds (only 3 required for the competition) and then asked him to back slowly out of the box. As Fat Pony is very submissive and tends to over-react to body language, I've been working on my breathing and keeping energy very low so that Fat Pony backs out slowly and doesn't rush, the aim being to keep him straight and avoid him knocking into the sides of the box on the way out (again to build confidence and trust). We repeated this until Fat Pony got more confident with the enclosure and then I put the end back on the box.
With the end on the box, I repeated the same exercise until I was fairly confident that Fat Pony had learned the Pattern of see box, walk in, wait 5 seconds, back out slowly. Once I thought that Fat Pony had learned that boxes are for standing in and not boxes are for jumping out of! I then did the same thing except I went along the outside of the box, with Fat Pony inside the box. I stayed level with Fat Pony's head, to maintain his confidence that I was supporting him and to give him a very clear signal where he was supposed to halt in the box.
We'll keep repeating this until I feel that Fat Pony is 100% confident about it and then I'll slowly start dropping back so that Fat Pony starts going ahead of me into the box and we'll build that up until I'm standing at the entrance to the box and sending him in.
A quite complicated, very clever exercise, this Box obstacle. Lots of components to it you don't really think about until you try it!