If drag reduction is your goal just REMOVE the spoiler and fill the holes. Even the spoiler you are thinking about has a lip that would probably add downforce, which is drag.
If you insist on doing something back there try to find something flat that extends the trunk without raised lips, etc.
Like this, but without the little kick up at the end:
The fine people at
http://www.edquay.com would gladly build you one.
