Input density structure for the reference spectral model (solid black lines): the inner lower-velocity component for the spectra on day 18 and 23 (SN 2018fip) and the outer high-velocity component for day six and nine (SN 2018fip-HV). The dashed black line shows the upper limit set by the spectral synthesis analyses on days 18 and 23 (see the main text). The density structures derived for GRB-SNe 2017iuk (Izzo et al. 2019) and 1998bw (Iwamoto et al. 1998) are shown by solid grey and dashed grey lines, respectively.