I've tried several different hard cases and have consistently used one
of the following with satisfaction.  In each case, their was not a
noticeable different in fuel consumption, though the appearance was
anything but aerodynamic.  I suspect that is because the Vanagon is
really not very slippery to start with.

1)  Rubbermaid footlocker (mine was a blue thick plastic with hinged
top);
2)  Large black plastic truck tool box from Home Depot (sorry,
manufacturer name         long gone).  This box was basically
rectangular, with a slightly inset, smaller circumfrence base.  I had to
measure several models before finding this one which actually BARELY
fits into the luggage area (front to back) and leaves several inches on
either side.

The trick to securing the box (or at least what worked for me) was to
purchase two large (about 2 inch diameter) eyebolts and install one on
each side of the box, using large flat metal washers (for structural
support) and rubber washers (waterproofing) which I made from pieces of
rubber from a plumbing supply.  Installing those about 3/4 of the way up
on each side and then attaching two heavy duty bungee cords on each side
(in a triangular shape; one to the front corner and one to the back
corner of the luggage compartment) seemed quite adequate.  Just to be
safe, however, I added two more bungees: one front to back on each
side.  I also used a bike cable lock to secure through one of the
eyebolts, as added security when the box was unattended.  We travelled
at interstate speeds and on winding mountain roads for about two weeks
on one trip and have driven through much rain...it didn't budge or
leak.

Hope this helps.