Best-fitting density enhancement model spectra (solid coloured lines) compared to observed spectra (grey) and the Si IIλ6355 region from the PV simulations (dotted). The faint regions of the model spectra correspond to the regions that are not constrained in the MCMC fitting. The asterisk by the −15.7 d spectrum of SN 2018cnw indicates that this spectrum was not used in the MCMC parameter inference (see Section 3.5).