Jim, if you're not height-limited by rules, you'd benefit a lot by having the end plates go up above the wing (ideally, with flat plates, almost as much as it goes down). Your high-pressure air on top of the wing is spilling off at the sides.
Also, you'd benefit by extending the leading edge of the end plates forward, as much as 30-40% of chord length ahead of the leading edge of the wing itself. And there's some benefit to having it extend behind, as well, but only up to about 10% of chord length.
This is all according to McBeath and his tests.
Finally, those thick uprights are strong, but anything you can do to make them more aerodynamic will help a lot with drag. In your shoes, I'd consider rounding off the leading edges and filing the back to a knife edge -- or wrapping the whole thing in a teardrop-shaped skin.
To show how big the penalty can be for even a round shape, here's an image from "Race Car Aerodynamics: Designing for Speed" by Joseph Katz.
