I simply removed the small rubber "feet" on my stock pillion and replaced it with suction cups. As long as there is a butt on it while you are riding it stays put with no problem at all. I've had several girls ride back there (some with no experience on the back of a bike at all) and they say it feels solid as a rock and as comfy as a stock passenger seat can feel. HERE
is a link to a thread with pics from a couple of years ago about this same topic.
Save yourself $150 and try it for yourself. BTW
, the mounting hardware that holds the rear seat on must be removed.