We present an experimental parametric study of spanwise centrifugal instabilities in an open cavity flow. We show that the mode selected at threshold depends on the cavity streamwise aspect ratio. For small aspect ratio, a steady mode is enhanced, while travelling waves are observed for large aspect ratio. The bifurcation is found to be supercritical for all configurations. Sidewall effects are shown to generate secondary flows that carry the vortical patterns. Spanwise confinement enhances the family of steady modes relative to the family of oscillatory modes. These results are discussed with respect to predictions from linear stability analyses and other flows developing centrifugal instabilities.