I'd start by studying some of the stuff that Phil Knox has done. You can click his thread right under "popular topics" on this page. He has been doing pickup aero for a long time and is always willing to give some pointers. I'm an Insight guy, so no real knowledge except, well designed sloped rear caps, wheel covers, and moon caps do seem to help
I also notice that there is a link to a commercially available rear cap in the "topics" but lots of folks here have done their own. Try using the search function to find some of those.