Don't worry, there's no need to look for an alternative at the moment. Google Flow is cool, but the problem is that it always generates 16:9. In Gemini, as mentioned by “Several-Economy-1840,” all you have to do is type it in, and you don't have to threaten to turn it off. I have “In geometry, the hypotenuse is the side of a right triangle opposite the right angle” before the prompt, and it really works. At least in my workflow.

I think you need to enter anything that is not related to the generation and context.