Hello @Josh.Dempsey
Welcome to the Autodesk Community! 🥳
I tried using CONVTOSOLID from your file, and... it didn't work.
@VincentSheehan has already found that redrawing the curve with LOFT works well, however, I have used another way.
In 3D modeling, especially solid editing, it is always easier to start from basic geometries as if you were doing it with plasticine, starting a general shape and removing or adding parts to reach the desired shape.
In this case, I have used LOFT to generate the surface of the corners:

Then, I have generated a solid from that surface with THICKEN to be subtracted from the rectangle.


After making the subtraction, I must select the solid and change the "Record" menu to "None" to allow me to correct the detail easily.

Once changed, holding down the CTRL key, I select the area to correct; some red dots should appear.
Now selected it, with the delete key, you delete and correct the corner.

Since it is symmetrical, I have cut the model in half, and then the piece is joined with a mirror.
You will have a better chance of success than reaching a solid that becomes complex from surfaces.
Well, it's longer here than in real life.
Successes, please let us know if it works for you. 😉
Johanna Esteban

Autodesk Expert Elite | Autodesk Certified Professional | AutoCAD Customer Council (Beta Tester for Windows & Mac)
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