Maybe you could kill 2 birds with one stone?
How about installing a cargo basket and then simply clamping a couple of 2x4's of the right length to the basket cross bars when you want to haul canoes.
You can always use extra luggage space and with the luggage rack, you have plenty of places to tie down canoes/ lock them up. Weight would be negligible and I don't think they'd hurt your gas mileage. Yakima makes one too.

  (You need to Login or Register