I made a backrest by buying a used two-piece Honda cruiser seat at the local thrift store ($10), and using the back passenger seat section as a a backrest.
I mounted this to a 9 x 12 piece of perforated aluminum that I found on ebay for another $10.
I had a Nelson Rigg tank bag that I then mounted to the aluminum plate.
I use bungie cords to strap this onto the luggage rack. It's a comfortable and functional backrest, acts as temporary trunk and is easy to attach/remove.
(A LOT cheaper than the $365 passenger backrest from Pirates Lair - that thing looks good, but I think the price is a bit of a rip.)
I can email pics of my contraption if you're interested.
alias Rob Osborne - Galax VA
** ** ATGATT **