This is a plant spacer for an Aerogarden hydroponics system.
If you're planting something which spreads, not all of the pods are used and it's necessary to blank off the spare slots. For some reason, the last kit I ordered (which required this) didn't come with the spacers. These are simple blanks which fulfil that purpose.
I'd suggest printing in PETG or ABS to make these dishwasher safe.
I am very much an enthusiastic amateur when it comes to designing. My designs tend to have two things in common
1) They serve a purpose. I like to design functional objects which have a real-world use. 3D printing for art is fine, but it's its real-world application which will take the technology forward
2) Wherever possible I design to be easy to print and try to eschew supports or complex printing; modifying the design where necessary and possible to achieve this.