I know one of the reasons they are so expensive is because of the insanely precise rigs they use to make them.
In college, we coated a piece of glass to make a mirror finish (Why was this part of the summer session in the Physics department, IDK) and it required a vacuum and putting a large voltage on a piece of metal (aluminum, I think) and the atoms would fly off and coat the glass. It made nice mirrors, but it was hard and we were doing it on like 1/2" mirrors.
But besides that, just getting it extremely accurately milled is going to be tough. There are different versions of “flat” and this would require a really “flat” setup, where flat in this case means exactly along that sphere.
If it’s interesting to you though, it would definitely be fun to try. There’s a good change you’ll get something that will work, although I don’t know how well. Since it’s so big, it would be able to catch a lot of light, but if it’s not perfectly “flat” it won’t focus well. Either way, it would be neat.