f you are interested in creating a cheap DIY planter for various reasons (my need was for a homemade aquaponics system), you can use a combination of the plastic downspout found in this link along with this 3d design to function as a cap for the downspout.
The design is quite snug if you're using soil, but you will need silicone to make it water tight for aquapoinics.
See the attached photo for how I'm using it in my aquaponics system.
The author marked this model as their own original creation.