No supports needed, but i reccomend 4mm brim. 0.2mm layer height is OK.

I strongly reccomend printing in PETG or similar material, it's easier to demold because it's slicker. PLA on the other hand is more matte/sticky and gets rather brittle after single use, because cement does chemicaly compromise its integrity.

When slicing, make sure all Z seams are at the side flaps and not in the actual faces that are in contact with casting material. Also make sure that there are exactly two loops of filament layed for all the thin parts of the mold (with no filled gaps). This depends on nozzle size, i've used 0.6mm, so the model is tweaked for that. Also it helps switching slicing engine (arachne/classic), sometimes one works better than other depending on other factors.

Before casting i like to wipe surface with rag dipped in paste wax (or leather care wax), very thin film ("barely greasy to touch") works best as mold release agent. I use binder clips to hold the two halves together while casting...

Mold is based on design ideas by @esparapse , but i've redesigned it completely in OpenSCAD and made the center cavity easier to demold by making it protrude a bit from the planter, so it can be grabbed by fingers or pliers and carefully liberated.