Belly pans would certainly improve the aero's of the Van.  It's even more
important how the air passes under a vehicle than how it goes over and
around one.  A van, being fairly far above the road surface, has lots of air
flowing under.  Smoothing the flow would work to lessen drag probably with
noticeable results.
  You could use aluminum sheets or even plastic and hold it on with Dzus
fasteners.  But it will increase the pain in the a** factor when you have to
work under the van...
I hated those belly pans, a standard item, on my Porsche.  Every time I
wanted to change oil or adjust some suspension I got an eye-full of grit as
I took off those stinkin belly pans..Running at speed without them
significantly reduced our terminal velocity however.
  Don Hanson