This repostory contains 2 folders ready to download which allow you to generate and decompress letterhead data for Wii Mail. This tool requires imagemagick to be installed for the generation script.
Place thumbnail.png, letter.png and (optionally) sound.wav into the "input" folder and then execute letter.sh in your MacOS terminal. Follow the scripts and it will generate a letterhead.txt that you can include into Wii Mail with WiiLink's nwc24 tools.
Rename the arc file to "input.arc" and place it inside the "input" folder, then exetue letter.sh in your MacOS terminal. It will display all decompressed files inside the output folder.