Since there is a safety issue for people, I'd recommend that you don't do it halfway.
I'm not clear as to how the drain is set up. Is it going through the side of the embankment? Or is it vertical in the pond? Or is it a catch basin on top of the embankment?
A quick goggle search found a lot of 12" grate covers. If you want to make something, maybe use angle iron or square tube with flat stock and bolt it all together. Or cut it all out and pay someone to weld it up.