Current opinion with the big snakes is that it is safer to feed in the cage. In the past it was believed that moving to a feeding box would get the snake to associate the box and not the opening of the cage with food. But then you end up with an enormous snake, that is very hungry and in feeding mode that you have to move between the cage and feed box. The best solution is to hook train your animals. You take a snake hook or another long device, and when not feeding, tap the snake on the head a couple times. I have a few young burms myself, and it does wonders.
I open the cage to clean or change water, and the burms are looking straight up at me like I'm going to be their next meal. I tap them a couple times, and they're immediately out of feeding mode. I've found it to be effective and my girlfriend and I even do it with our ball pythons. The most important thing is that you must remember to be consistent with it in order to get the snake to associate the tap on the head with the absence of food.