Edit: this is pretty much what you already did:
I think you might be better off with a truncated pyramid; rather than a half cylinder. Taper it at the maximum angles (15-18 degrees on the top and sides, and 6-8 degrees on the bottom?). If the trailing face is crisply truncated it is much better for the air flow. The curve is better than nothing, but it still has a messy turbulent wake.