Maybe this will help the the Volvo SW
 

I got plastic I got tape I got beer. Anyone think this will help? or am I about to get a littering fine for flying debris on car. SC

looks like pic no work , 2nd try

ok

I suggest you start after 4:20
You may end up with the " Red Green" effect, if you attempt while drinking beers.. :-)

Remember if the women don't find you handsome, they could still find you handy..

keep your stick on the ice

Ok lets try to post that there picture again, Yaeh! SC

maybe
 

The base pressure that you're trying to increase is governed by the pressure at the point of flow separation.
Since the spoiler is only partial coverage,the low pressure all around the perimeter of separation is still being communicated into the wake.
For the extension to really help,it needs to wrap all the way around,forming either a cavity,or closed in to make a proper boat tail.
Also,the downward slope is 'fast' and can lead to separation and attached longitudinal vortices which impart really high drag.
Check out the 'Aerodynamic Streamlining Template' tool at the top of the Forum page as a worry-free guide for shaping.