Hi, Daniel.
I think this depends on what type of simulation you want.
If you want to simulate the "overall" cross-section effect over a considerable beam length, you should use line elements (of the beam type), and input those cross-section properties (area, moments of inertia, etc.). In your sketch, under the inspection tab, there's a "region property" tool that measures some of those for you.
However, this won't be able to measure the details of your model, such as stress concentration at the corners, only the overall effect over a big length.
The other way I can think of, in case you want to analyze those corner stresses, is for you to simplify your model and add solid elements. A 1/4 model for this would be ideal (such as the first quadrant considering the center point), with symmetry constraints on the horizontal and vertical sides. This tutorial describes how this can be done:
https://blogs.rand.com/manufacturing/2019/12/using-symmetry-constraints-to-reduce-the-element-count-...
If you are having trouble, you may also share your assembly/parts here, in a compressed .zip file.
Best regards,
Leonardo de la Roca
