Too small for 20.3 and I'm not sure about availability.
I wasn't suggesting your solution was anything but perfect.
Just added the TD note for those who don't have an existing number plate on the smokebox!
Availability is fine. David at Valley Brass responds to emails and ships promptly, in my experience.
TD-19 is 13.13 mm diameter which is about 1/2 an inch.
TD-71to73 have the Baldwin name around the outside, and are 15mm.
TD-139 & 140 are also plain and larger. The largest is 18.75mm, or about 3/4".
TD-198 is the raised brass numbers, 5mm tall.
I used them on a loco a few years ago.