Discover the 1 x 4 x 2 Window, Red or White from the Supplemental collection - 9 pieces of building excellence.
Let's Build It! compares 1x6x2 bow bricks, finding part 15254 the best fit for the 1x4x1 2/3 window with arch (20309). They note part 3307 is too tall, while 12939 has a noticeable gap when used with the window.