Here's my take, enclose the front, let the body taper on the sides, get the area behind your body and head into a taper, it's the best you can do for the simplest design. You'll get 85% of the drag reduction out of this, to get the other 15% you'd have to go to real extremes. Try to get as much air around the sides as possible. I have not been able to illustrate my idea about the front, you basically want to have the "Hood" stay high out as far as possible to push the air around you, the driver.
Sort of this:
Here's my drawings....such as they are.