Initial ejecta properties used in our simulations. Top: ejecta density structure versus velocity for the original Type II SN model at 10 d after explosion (black). For this work, we introduced a dense shell at 11 700 km s⁻¹ (blue), which we clumped (maximum compression factor of 100; red). Bottom: corresponding mass fraction versus velocity for H, He, O, and Si, as well as for the adopted (normalized) shock-power profile associated with the interaction between the CSM and ejecta.