Some time ago, I bought plastic terracota flowerpots from TESCO (with label referencing terracota.hu). Though they are standard 13 cm in diameter on the top, they are narrower at the bottom than usual, so the regular planter inserts don't fit.

I used the spare ends of PLA filaments I had laying around to print the inserts I needed. I'm sure you can buy compatible inserts, but I didn't have anything else to do with the filament anyway.

Printed at 0.28 mm DRAFT with Generic PLA preset on Prusa MK4S. 33.4 g & 41 min per print.