I have been using a Thule roof rack for about 10 years to carry my canoe. It
mounts on the 'rain gutters' and is specifically designed to provide plenty
of clearance for the Westfalia pop top roof. The vertical height of the
cross bar can be adjusted to your liking although the bottom most position
is still a tad high, forcing you to stand on your 'tippy toes' (if you're
under 5' 9" or so) to hoist the canoe 'way up there'. If you position the
rear rack just right, the roof can be raised without having to remove the
front or rear rack.  I also installed bolts to prevent the canoe moving
laterally along the bars. Thule makes top quality stuff albeit somewhat
pricey, but hey, it's for your Westfalia...it's worth it!  One more thing,
buy the Grumman canoe tie down strap system to secure the canoe to the
bumpers. They're easy and quick to attach/remove and are very secure.

Roy O.
'87 Westfalia
West St. Paul, Manitoba, Canada