The best setup for a luggage carrrier is to make a plywood [very thin]
deck and keep LIGHT bulky things inside. If you solo WW it is helpful
on the river too!
  A big canoe on a bus and slightly less so a vanagon is very exciting in a crosswind!
  The best setup is to use Q&E racks (25$), putting the rear one
as far back as possible ,mine is 6" from the rear. cover the rear one 
with indoor outdoor carpet. load and unload by putting the front
of the canoe on the rack, rest the rear on the ground,
lift the rear and push it forward onto the rack-EASY!
 I carry my 17'4" intrepid on our vanagon and, before that on our
kombi exclusively and without incident.
  I remember running one shuttle with, because we were lazy,
 4 canoes and 2 kayaks tied loosely on our kombi [ 2 upside down
and 2 right sideup on these with the yaks wedged in somehow].
someone jumped out in front of us, i slammed on the brakes and 
one top canoe went flying on a trajectory to land
in the lap of the cars passenger. I had a great view of the cars passengers 
primal scream. Luckily the rear of that canoe was tied to the rear
bumper, i'm sure it was by accident, and it stopped anout 1 foot from the
cars windshield.
 it was a good lesson about not hurrying!

 Bill