Retro fitting rear parking sensors?

[GrahamS4]

So the bulbous **** end and limited view of the rear on cabriolet always makes reversing interesting so I'm looking at fitting rear parking sensors and wondered who else ash done this and which route did you go. Did you replace the rear bumper with an OEM and associated parts or has anyone gone down the route of having these fitted?
http://cobracartech.co.uk/en/front-parking-sensors/969-front-parkmaster-4-sensors-flush-mount.html