PowerPoint - Creating a Custom Shape

Hi all, here is the backdrop for my question: I have a slide in PowerPoint that I have put an image over the entirety of the slide to act as the background. On top of the image, I have placed a partially transparent oval that covers the bottom 2/3 of the background image. I want to fill in the upper 1/3 of the slide in a solid color that will cover up the background image without blocking out any of the image that is covered up below by the oval. Is it possible to create a custom shape that forms to the curvature of the top of the oval's line and the top boundary of the slide so that I can fill in that shape with a solid cover without having the new object infringe on the boundaries of the oval?

Thank you in advance if you can offer any help!