My Hitachi job site saw had a weird goofy style and size throat plate.
The only thing I could come up with as a ZCI, was to cast a copy of the plate in plaster paris, then use that cast as a negative - and cast some actual plates using machinable wax, then trim those down on the outsie to fit - using the router table as Glenn shows above.
Machinable wax is a very hard tough material. You can buy it or make your own.
"Life is what happens to you while you're busy making other plans." - John Lennon