I don't think 'Knock Out' roses would do well in such a shallow window box. If you want to grow them, you would be better off planting in individual deep containers where they should be fine. You might be able to grow small miniature roses such as 'Rainbow's End' in the window box. Other suggestions for the window box would be tender perennials, often listed as annuaks, such as hybrid fan flower (Scaevola), verbenas such as the Superbena? series, petunias, calibrachoa, and Marguerite daisies (Argyranthemum). There are many new and exciting tender perennials on the market today so mix and match for a long-blooming and colorful display.