This is an idea from Jasper K. in Flagstaff, the buyer of one of my vans.

He didn't want to mount a Yakima rack onto his Vanagon using the
Yakima tower extenders so he went to the local hardware store and got
a bunch of stainless steel washers and longer bolts.  This is the
result:
http://www.californiavan.com/yakima/
A simple and effective solution.