It is just an 0.25" thick, 3.5" × 3.5" square with a single shape cut out of the middle of it, like this:
I have a CAD file (.DWG) for it that has been used before to make that piece, but I need another piece made. Also, I want the cuts to be at least as smooth as the surface of the steel already is from the mill. With the one I had made several years ago, the dimensions were fine but the cut edges had little striations in them, like this: