Resizing a firebox on a brass engine

Hi there. There’s a guy that used to post a lot here that did this using styrene. Here’s one of his posts :

https://forum.trains.com/t/lehigh-valley-2-8-2-scratch-bash-quick-look/272611

and

https://forum.trains.com/t/reading-n-1-2-8-8-0-kitbash/273973

Simon