You have the wrong breaker! The breaker you purchased (MBK200) is for replacement use in older loadcenters. The MBK200A is a completely different design. Look at the breaker picture just below the breaker you currently have in the link you posted. It’s marked EQ869X.
Modifying the cover would be the least of your problems trying to use this breaker. How would you connect the incoming feeder conductors? The breaker you have is designed for bussed connections on the line and load side.
Curt
