Widow boxes / rectangular planters (Brunswick BN3)

I am looking for 2 or 3 window boxes which I can use to add a bit of colour to the windows of my new flat. They would be the type that sit outside the window inside a wrought iron rail (see attached picture.) Either wooden or plastic would be fine - any size up to 115cm (L) x 15cm (W) x 25 cm (D). Thank you. D