Blanc, I've looked for a cover for the back of my r850r to replace the pillion saddle. I never found one. I do share your interest in a cafe racer r850r though
I've had this for several years now.. not being made anymore. but shouldn't be too difficult to have one made for you.
It sits on 4 rubber feet.. has two hooks per side that locate into the slots on either side and a steel loop that uses the rear seat lock. Is very secure and spreads the weight of anything you care to strap onto the space.