To be completely frank, that sounds like a bad idea. Without the aid of a very large milling machine, itd be near impossible to maintain a consistent depth all the way around, and the slightest misstep would create a rather unsightly gouge in the top. Adding to that, its been a while since i checked my say, but i want to say the lip that the insert sits on is itself rather thin most of the way around, save for the area around the leveling screws.
For what its worth, ive got the exact same saw, and i made a ZCI for mine doing exactly what you said, cutting to shape and rabbiting the perimeter to fit the existing hole. Its been maybe 6 or 7 months now, with a pretty good amount of use, and its holding up great. Even though there is only about 1/8 of material holding up the insert, the insert really only needs to carry its own weight, its not likely to see and significant load, so 1.8 is more than sufficient
If the idea of a wood insert still sounds iffy to you, why not make a metal one? If you have access to the tools, id wager you could pretty simply make an aluminium insert. Heck, you could probably make an aluminium insert with just woodworking tools