Have you looked at the Garhauer mast plates? For these low prices, it is hardly worth making your own, if they will fit!
Garhauer Marine Hardware -657583
But, they are all rectangular. I just removed my mast step today, to do the same thing. Since my step, and the raised fiberglass area underneath it are both oval, I think I will have a machine shop make one. I think that a rectangular plate would overhang too much at the corners, and perhaps be a bit weak, as they would not be suppported as well.
Of coures it depends on loads, but for a typical cruising 30'er, anyone know how thick a plate should be done in stainless? What about aluminum, how thick?