Sorry, but no amount of aerodynamic shape will help you at 0.5c. The fallacy of the argument of an angled impact reducing the kinetic energy of a collision is that it requires an inelastic collision, ie, the particles hits the top layer and bounces right off. At relativistic speeds, a particle is going to go right through the top layers of steel like it was sweet cream. Any known solid at these speeds in an impact (dust, pebbles, a baseball) can be treated as just a collection of particles because the kinetic energy far exceeds the binding energy of the individual atoms in the particle. So a particle won't bounce off a plate like a poolball. It's going to hit like a drop of water splashing into a film of oil, i.e completely disrupting both structures and mixing them temporarily. The result is that whether the shielding is angled or flat on, each impact will vaporize approxomately the same volume of shielding.
An analogy of this effect is in the study of hypersonic impacts of solids done for explaining meteor craters. A roughly spherical meteor impacting a surface (and this holds true for micrometeors hitting the lunar surface at only 20km/s) will leave a round hole about 10 or so times its radius whether it impacts at an angle of 0 or 60 degrees. Craters will only elongate enough to give a clue of the angle of impact at very shallow angles.
Back to your ships, an aerodynamic shape is worse for high speed STL travel than a long thin cylinder. This shape will prevent a minimal cross section for impactors (and at 0.5c, they'll all basically be hitting head on). Armoring the front of the cylinder with a bunch of ablative ceramics or what have you would be the most ideal with present technology.
Also, the density of the solar system space is about 5 atoms per cubic centimeter. Interstellar space is less, but not negligable and since the journey to the nearest starsystem would take about 7 years or so at 0.5 c, you'd need a fair amount of plating to grind off.
Some type of repulsive energy shield you could beam in front of your ship would be ideal. Honestly, glueing an iceberg to the front of your ship would work pretty well too. But my main point is that aerodynamics work in classical fluids, they don't work in relativistic space flight.