The easy but expensive solution would be to add an aftermarket rack
system with yayak cradles of some sort (there are several variations of
cradles for kayaks).

For starters go to:

http://www.rackwarehouse.com/

and just follow the fit guides from Thule and Yakima for your make and
modle year van.

Which kayak cradles are "best" is worthy of another post query on rbp.