The whole window will have a one foot wide window sill. I will have to show you the place to get a good idea of things. Basil planted in teacups for the kitchen, cheap and pretty teacups planted with fresh herbs on the window sill.
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.