So as long as the tray is on screen it's fine, no need for clamp then;
Also I didn't suggested the 8-Direction to fix the box falling problem, only to make the movement management more streamlined, to fix the said problem, the pin behavior should fix it, by checking when a box collide with the tray or another box, pin it to the character position;
However there is just one tiny problem, without the family feature, there's no way to check if a box collide with another box correctly, because C3 wouldn't know which one to pick and pin to the player, there's a solution however : have an invisible object that is placed on the tray and follow its X coordinates, that would check when a box touch the top of the pile;
Each time a box touch that checker, it is pinned to the player, and the checker is moved a few pixels up, do you see what I mean?
EDIT : Nevermind I though of a simplier solution, what do you use to make the boxes fall?