Switching Caps in 8 3/4 Case

I am helping a friend with his 8 3/4. He has lost a cap from the center section. How likely will another diner cap work on his case? What I have heard is it is better to start with a new case. I know there are aftermarket caps available so it can be done. Cost of new caps may steer him to another center section.

Any experience with interchanging used or new caps is appreciated.