We present a systematic investigation of the in-plane uniaxial magnetic anisotropy induced by the morphology due to ion erosion of MgO(001). Ion milling at oblique incidence forms a ripple structure on the MgO surface the grooves run along the ion beam direction. Ultrathin cobalt films grown on such templates show a dominant uniaxial magnetic anisotropy with the easy axis along the ion beam direction. Both the strength of anisotropy and its symmetry can be controlled via the milling conditions, allowing one to fine-tailor the anisotropy in magnetic films. A uniaxial, volumelike anisotropy contribution is found, which is explained by the modulation of the magnetization perpendicular to the ripples that causes an increase of exchange energy.