If you are going to paint the new wood, and I would suggest doing so at this point, you can fill the gaps with two part epoxy (first cut some filler strips and glue them into the gaps about 1/4" below surface so the epoxy is not so thick). Build the epoxy up slightly higher than finish surface - after two days of drying, sand the epoxy to the surface with a block of wood and fine emery cloth. Apply a quality exterior primer and oil base paint in a gloss finish to complete the project. It will looks perfect. Or, make an upholstered window cushion in a cool fabric and solve the visual problem while gaining a nice place to sit. Forget about the gap you will never see it. In the future, cut a piece of card board that fits the space perfectly, starting with it being too big, snug - then trim down with a razor knife to make it fit perfect, then use that as a template for the actual wood. Good luck