CustomFlagLoader
LGPL v2.1
Screenshots:
Download
Source
Installation instructions:
Install BepInEx
Download & extract the mod over the top of the BepInEx install
Place your custom flags (.png files) inside of a new folder "Kerbal Space Program 2\flags". The plugin will create the folder for you on launch if it doesn't exist
The flags can be any size; they will be stretched accordingly.