I have a similar thing in one of my quests, you can look at this, it might help. This works fine for me:
function deSpawn()
TOP_LEFT_CORNER = {x=1154, y=978, z=10, stackpos=253}
BOTTOM_RIGHT_CORNER = {x=1167, y=990, z=10, stackpos=253}
for Py = TOP_LEFT_CORNER.y...