Genius slipper design where you knit a short "scarf", fold and sew onto your bottom. Garter stitch extra chunky yarn about wide, and however many rows you need, then sew into the 2 layers of the felt bottom.
Pour casting concrete into rubber gloves, being carefully to remove air bubbles from fingers. Mould into position working fast before concrete sets. When set, peel of rubber gloves revealing your concrete planter.