Try looking at the shapes that snow gets drifted into against other objects besides just on cars. And also take into account the liquid density of snow. Wet snow will pack and freeze and the shape we see may not be so much the result of aerodynamics as of imobility. While dry powder snow flows quite nicely and should leave some interesting ideas to test.
Yes if your car aero is perfect there would not have any snow left after test. I would be interesting to see for example aerocivic do the similar test. he has pretty good Cd but pretty rough surfaces. So the pockets should fill with snow.