the boys box is already on most fields here is what I suggest, Stall warning only in effect in last 10 minuets of a game, warning in effect when it is apparent that the possessing team is not trying to advance the ball for a shot from anywhere on the field, from onset of call team has 30 seconds to get the ball into box, and keep it there, if the ball or the player carrying the ball leaves the box it's a turnover. The box should be 20 yards from the center of the goal in towards the side out lines, 5 yards back from goal line extended towards the end line, and extending all the way to the restraining line. A shot clock becomes too suggestive. Are they trying to advance the ball? once in the zone, are they really working for a shot? if a shot is taken was it really to score or was it purposely missed to reset the clock? With a small box it is more cut and dry, the player/ball does or does not leave the box. The only suggestive element is, is the team trying to advance the ball prior to the stall call.