Take an old ice cream chair and sand off all of the rust. Mine came from the trash but you can also pick them up at yard sales for a very low price.

Take your flower pot and trace the size around the seat of the chair. Cut the seat. Some chairs have wire mesh seats. These are best to cut with heavy duty wire clippers.

wrap metal tape, found ast your local hardware store, around the cut edges of the seat. This is only to cover up the sharp edges of the mesh after your cut.
You can skip this step if you wish but be careful of the mesh edges when you remove the flower pot to replant.

Primer and paint the chair. Be sure to use an exterior paint.
Allow to dry.

Place your flower pot in the seat hole. Fill with dirt and flowers and don't forget to water!