I've been lurking for about two weeks, but I have experience on this one.

I have used foam pads (fitted to the canoe's gunnels), straps across the top, and tie-downs front and rear to carry my 85 pound Old Town Discovery on top of my 84 Westy.  It worked fine over highway and dirt roads, but requires some umpff on the straps and height (I'm 6'3") for managing straps over the top.  Most canoe/outdoors shops carry a set of foam pads and cam straps.  You'll probably need more for the front and rear tie downs.  

A Westy-type after-market van conversion shop here (Denver) sells plates that can be bolted to the sides of the poptop and a Yakima rack mounted in them, but I'm leery of popping holes into the poptop on purpose!  I also think I saw these at a Yakima Rack display or in their catalog.  Has anyone used these?  I have other stuff to haul than the green barge boat, and I wonder how well these plates work under a load of 100 pounds.

By way of introduction, stats on our rig.

Our Beast: 84 Westy GL Wolfsburg edition (what does Wolfsburg edition mean?)

Our Beast's Name: Sisu (Finnish, roughly translated as extreme intestinal fortitude)

Our Beast's Field Markings:  Two-tone color (butt-ugly brown & butt ugly beige, AKA BUB-squared); usually dust camoflaged; known to veer wildly whenever birds are sighted.  A Westy is probably the best birding vehicle/blind ever

Ed Furlong
Evergreen CO