When I tried to work inside the smokebox, it was pretty tight.  If I ever needed to work on the throttle valve, I would have to take the smokebox off which would be a major undertaking.  I decided to cut a hole in the smokebox to make things easier.  Then I made the covering/door.  The smokebox is 3/8" thick and does not cut or form easily.  I made the cut on top where it will be hidden by the skyline casing.  I used a single bolt to hold it in place.  You can see how it latches in the second photo.

And here is it installed.  Makes it a ton easier to get to the plumbing and valve in the smokebox.