>> If for some reason you cannot build Emacs or work with master, please
 >> tell me.  I'll then try to explain how to get the desired behavior with
 >> Emacs 25 and your .emacs alone.  Meanwhile closing this bug.
 >
 >    I'd like to know how to do this on Emacs 25.

Store the attached file my-fit-window.el somewhere on your machine,
byte-compile it with Emacs 25 and load my-fit-window.elc from .emacs.

And please tell me if it works.

Thanks, martin