Tik recommends:
===================
Sort of a big square frame on top of your van. All this needs to be
attached to the existing rack. If you have gutters, I suggest using
turnbuckles and strapping to make a tiedown between the extended cross
piece, and the gutter. This will keep the front of the rack from trying
to lift off while driving down the road.
======================

Thanks Tink. After my last post, I got to thinking about what you and
Michael had suggested and I realised that a frame/box was what you were
getting at.

Clearly I'll have cross pieces fore and aft (as far in either direction
as possible). I'm also thinking about adding another cross piece
somewhere in between, near the center of the hull. Or do you guys think
that would be overkill?

I'm also getting the impression that most people don't care too much
whether the cross pieces reflect the curvature of the hull. I was
thinking at least a bit of a "V" (keeping in mind that the load should
be carried by the keel, not the sides of the hull -- so a wider "V" as
opposed to a deeper one). Again, is this overkill?

Finally, Michael, I think, recommended 2X6's for the cross pieces. That
makes sense to me in terms of "meeting" the upward hull curvature of
fore and aft sections.

So, if I can just get you guys to sign off on this project, I'll be off
to the lumber yard.

Cheers,
Wilf
===========