[aerohead]

* the flow extender will not be exposed to significant aerodynamic loads, and some tape systems adhere so strongly that, you might destroy the device whenever you attempt to remove it.
* a lanyard, like Chrysler Corp. used on their 1969 Dodge Charger Daytona Coupe rear wing, to ensure it wouldn't strike another race car should it be knocked loose in a race, might be enough 'insurance' for your device.
* other than some structure which is captured within, and held steady in the gap of the rear hatch, from which the 'wing' could be attached to, I don't know of many options for sturdy attachment that wouldn't damage the Volvo's paint.
* I don't think you want to drill holes in body parts, and then run fasteners 'out' through the hole, into a female threaded insert, bonded to the 'underside' of the 'wing,' with a weather-proof gasket, like carmakers do.