|I have a Thule rack (http://www.thuleracks.com/) that came with my Van.
|It's designed to mount to the rain gutter and go over the pop-top.  The
|advantage of this, of course, is that you don't need to drill holes in your
|fiberglass top.  The disadvantage, of course, is that you can't pop the top
|while the rack is on the van.

The Thule rack that I use to carry a canoe on my Westfalia sounds just like what
you describe. I have no trouble popping the top with the rack on the van. Just
position the rear rack far enough back on the rain gutters and the top can be
popped without hitting the rack crossbar.

Roy O.
'87 Westfalia
('till death us do part)