A feeding box sounds like a fun idea. The great thing about building your own habitat is you can do whatever you want. We have been flopping back and forth between feeding inside and outside the snake's enclosure. Although I am not worried about getting bitten. The key thing there is to make sure you have no rodent scent on you. I am curious to hear if your snakes will use it in a timely manner. Ball pythons seem to have a bit of cat in them when it comes to eating or moving around. They take their time and don't want to do what their owners want them to do. But if it doesn't work out you can always just remove it.

From your description of the terrarium I could not tell if you have a temp gradient? The snakes need different temperatures within the tank to be able to regulate their internal temp. It sounded like you were heating the floor evenly. Not sure if you were just talking about the basking area which technically you don't need but given that it is only 12 inches tall an adult snake will probably think it is a nice warm roomy underground den