I bought a grab rail that mounts to the petrol tank bolts, much safer than rear grab handles as the pillion can brace themselves on both braking and acceleration Will take a photo later if you'd like?
Ron Haslam Race School uses these. Not sure that the Petrol Filler Cap bolts were designed for that sort of stress though!
Didn't get chance to take photo of mine but got the link for a company that does the same grip with sample pic Pillion Tank Grab Handle for most makes View attachment 15333