Can you measure the old one? Is it a flat gasket or an O-ring? I used to know of someome who could make any gasket you wanted. He even sold the tools (expensive!!)
to do it yourself. I'll see if I can find the contact info. Just a guess here, but from studying the case, it looks like it is a flat gasket with holes for the screws. If that's true, it might be less expensive to see if an AD can replace the gasket than to have one made.
This is one more reason I don't take high end swiss watches in for repair. The parts are just not available outside the closed sales and service environment established by the various manufacturers.