It’s difficult to understand exactly what you are after but I suspect you should be able to select the faces of your rectangular prism and then use Intersect with faces to create the required edges. With that done, erase what you don’t need.
My goal is to do as shown in the 3rd picture, but it does not look fine as the 90° corner, goes inside the main form. The best would to measure the radius (I do not rember it) and I could create a radius in one the corner of the reactangle, and then move it at the bottom.
You can measure the radius if you want but it would be faster and easier to do what I told you and use Intersect Faces. That way you wind up with a perfect fit.
If you have Bool Tools 2 you can make both objects solids and use the outer piece with the radiused surface to trim the block. If you want to combine the two into a single object you could also use Outer Shell.
If this is for 3D printing you really don’t need to trim that block at all. It can be left as a separate component or group.
