Based on sketchy polygon style but for lines. The random points are also random in size. You. can manage the distance from the line edge by altering the offset calculated symbol attribute (set by default to 2.5):

array( 0, randf(0,randf(0, 2.5)) )

Points are also guaranteed to fall inside the polygon and randomness is contrived so that you don't see a 'stripe' effect along the polygon boundary.