take any horizontal square pixel image and divide by 2.39.
An image with an aspect ratio of 2.390 and a width of 4096 has a height of 1714: 4096×1714 (4k)
An image with an aspect ratio of 2.390 and a width of 3840 has a height of 1607: 3840×1607 (UHD)
An image with an aspect ratio of 2.390 and a width of 1920 has a height of 803: 1920×803 (Bluray spec)
Subsequently, you can create a new project with this 2:39 dimensions above and paste your timeline into it. Select keep existing settings and it will basically put a window over your footage like a center crop. Don’t worry, the full image size is still there underneath, so you can pan, scan, and reframe. Only it’s a 2:39 window and project so no messing around with a matte and then remembering to crop on export. Hope I explained it well enough.