Please do not plant directly into a planter set.
Planting direct will block the integrated self irrigation system which may result in plants drowning or dying of thirst.
The great thing about using potted plants in a Wonderwall system is how easy it is to change a planting scheme. Simply remove the pot and insert into another planter set, or take them indoors to harvest yummy edibles.
We recommend using a standard 1L garden centre sized potted plants, or...
Alternatively pot own. The perfect sized pot (1L = 13cm diameter top x 9.5cm diameter bottom x 11.4cm height). It is difficult to find this exact size, so below is what we recommended to use.
Future Gardens supply them (Wonderwall is not affiliated or commissioned for this product recommendation):  https://www.futuregarden.co.uk/round-pots