"Easiest", and "most cost effective", are two different methods...

Easiest: Buy a full quarter panel, cut the piece out that you need, and weld it in place. Discard the rest of the quarter panel, store it for possible future use, or even try to re-sell it.

Most cost effective: Buy a piece of flat sheet metal of the correct gauge, cut/bend/shape it to make your own custom patch panel. This is only cost effective if you are fabricating yourself and your time is free.

I have never seen a patch panel sold of the piece you are asking about.