I sent this to somebody but I did not keep a copy and I did not cc it to
Vanagon.com so I will repeat it here as there is more interest in this than I
thought.

One of my ideas is to turn the two front seats to face each other remove the
head-rests, make two metal/wood cross bars (brackets) with two prongs (the
size of the head-rest metal prongs) that will go down into the head-rest holes
in each of the seat tops, this is to keep the hammock in place.  Then make
two metal/wood runners to go the width (left/right) of the van from which one
could sling the hammock attach these to the brackets on top of the seat
backs.  One could figure out spacers to raise the hammock a bit higher, if
need be.

Here is a  diagram of my idea (on my screen it looks OK,  you may have to
toggle a font to get to look OK.

driver's side                                                           passenger's side
[=======================================]
[                                                                                                               ]
o--------------|                                                                        |----------------o headrest holes
[seat       |                                                                   |seat           ]
[                   |                           canvas                          |                       ]
[--------------|                                                                        |----------------]
o                                                                                                               o headrest holes
[                                                                                                               ]
[=======================================]

This could be an upper bunk with the lower bunk being a padded board
placed on the seats.  Enjoy,  Malcolm S