I printed this insert:

https://www.printables.com/model/157852-coup-boardgame-insert-also-fits-reformation-expan

However, I did not measure beforehand, it did not fit in the box for my copy of Coup :( 

So I made this lid that goes around the insert. The tolerances are pretty tight - you may want to slightly scale X/Y dimension to allow more clearance if you don't like the lid to be “sticky”.