I wouldn't really worry about feeding him in a separate feeding container.
"Kill box" sounds ominous... never heard anybody call it that before.
Although there are reasons to feed outside the snake's home, such as risk of impaction from swallowing substrate, the only reason I've found to matter is personal preference of the owner and snake.
I feed in a separate container because it is easier to clean out the vivs with the snakes removed for feeding. It also keeps mouse guts from getting all over the viv.

Also, it means I physically inspect each snake as opposed to just tossing food in.
I have heard of impaction happening, in baby snakes only, but I have never seen it or personally spoken with anyone who had experienced a case of it.