M_CSM as a function of the r-band absolute magnitude M for different assumptions of t (panels), T (shaded region), and F_R (colours). The five objects for which we estimate M_CSM in Sect. 4.4 have been marked in the panel that has t the closest to the detected signal. Decreasing shell thickness and temperature to mitigate M_CSM is effective, though there is a limit to how far this can be done while keeping realistic values for F_R.