Is it possible to save a winbox custom layout?
Yes.
Things are saved in ‘Sessions’
Usually it is a unique session by IP address you are connecting to, and it auto saves by default when you log out
So when you log back into the same router - It will load what you had opened when you left.
I prefer to have auto save OFF - and I logged in to a test router, set up my windows how I wanted them for different router types, and then went file → Save As to save the view as a custom sessions (‘viw’ file), then in your managed list of routers you can set the "Session’ parameter to what you want, so with autosave off it will always load the same screens in the layout you want.
For example, I have a session for CPE (customer routers), BGP (Core/BGP routers), PPP (User Access routers) so they all display slightly different.