--The item box only activates when the player presses Select, even when getting hit.
--The powerup is granted instantly instead of falling from the box.
The newest version (1.3) swaps powerups properly, forbids activating a powerup if the player already has it, and improves a bugfix for preventing item box activation during special player animations, such as dying, getting hit, and powerup transformations.
Full credits and version history are in the file.
Note: If you're planning on using this with the Perfect SMB3 Powerdown patch, patch this first. The reason being, this disables the item box's auto-drop routine, whereas the powerdown patch hijacks it.