Actually one of the guys over east on OzBB made up one himself a little while ago using a car headrest
QUOTE("chasg")
Made a backrest today using a Honda car headrest,some 6mm flat steel, tube protection rubber, hardened nuts and bolts and a welder. Haven't taken the wife on the back yet for a test ride but seems OK.

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QUOTE("chasg")
It is fitted down through a 15mm x 40mm cutting in the the tail cowl.This can be blocked up later using a blank rubber grommet if l decide to take the headrest off.I basically welded a 6mm plate across the tail end of the frame, welded an extension to the head rest and bolted the 2 parts together using 3 hardened bolts and nuts for ease of removal.

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QUOTE("chasg")
After going for a ride yesterday,had to adjust the backrest to tilt back more. Also changed the covering of the headrest stem by using a push bike inner tube and a large rubber grommet. See my photobucket for latest pictures.
Now playing around with an ideal for lowering the rear pegs. Will make a template out of timber. Then if it works get a set made out of metal or aluminium.
Not my kettle of fish but he reckons his missus likes it