I am taking a Bezier curve, turning it into a mesh, then mirroring it on the X axis and joining the two meshes, then I mirror it on the Y axis and join them. I merge the vertices where the joints were made, then create a face and make a prism out of it. Then I duplicate it (part of my process to be sure I keep the original one) and move it so it goes through a cylinder and try to apply a boolean modifier (using the Difference option). But the boolean modifier does nothing.
The lower left item is the bezier curve I'm working with. (The original. I created a duplicate to work with.) I turn it into a mesh. From there, I have tried duplicating and mirroring with CTRL-M and I have also used the Mirror modifier. (In both cases, of course, I have to use the 3D cursor to change the object's origin so I can do the mirroring properly.) I have tried this mirroring the X axis first and the Y axis first.
I have also varied creating the face or faces. I have created a face once I have one mesh going all the way around, then extruded. I have also extruded, then created both the top and bottom face separately.
No matter what I do to take this Bezier, convert it to a mesh, create the shape I need by mirroring it twice, extruding it, and adding a top and bottom face, once I move it into place and try to modify the cylinder with it so I have a hole in the cylinder that matches the prism shape, it will not work. Nothing happens and the cylinder is not modified at all.
What am I doing wrong and why won't this prism work as an object for the modifier?
(Note - adding the .blend file once I have the URL for this question for Blend-Exchange. If there's a way to do that before posting the question and editing later, I'm open to it.)