[LWJ]

I used to sell aluminum parts by the semi truck load. That is a big casting! Big = costly.

Milling isn't cheap either.

I don't have a basis for cost estimate on this other than to say expensive!

Can you fill in the port light opening or expand it for a standard size? Probably cheapest.

Do you need opening portlights? If not, you can do custom pretty cheap. I supplied aluminum to a custom sized port light manufacturer long ago.

This is a sticky problem and takes creativity. Last thought is weirdest but may actually work. What about 3D printing?

Good luck! Show us more of the project.