I live right near Oregon, where it's a law that your gas must be added only
by the station attendant.  So, explaining or showing how to close the gas
cap became kinda tedious, given that many station attendants are a bit
mechanically challenged, or perhaps they aren't really concentrating (for
minimum wage, who does?)
  My fix was to use a black zip tie, drilled into the gas cap's rim and
through the body work.  Then I took one of those parts marking paint
pens..(handy little gadgets) and made some arrows on the cap and on the body
work..When the attendant gets the key, he/she inserts it and sees the
surmarks, or arrows lined up, and usually has no problem remembering how to
duplicate that alignment when closing the cap.  The wire tie keeps the cap
and key attached to the van at all times..No more "left on the pump" gas
caps.
  Be very careful when you make a hole for the zip tie.  Gas fumes do
explode with a spark..
Don Hanson