Not sure what you are building
if it is a Kamm back - just make it gently curved
metroMPG has a good thread going on this right now
no magic - just the longer the better
if you are building a spoiler and not a Kamm back
you might find this interesting
about a third of the way into the infomercial they show a back view of this concept SAAB
looks like it has active spoilers that extend behind the hatch and under the bumper - but they seem to indicate they are for low drag
been thinking of something similar myself
interested to see what you are building