This works now:
{
"name": "your-company/your-project",
"require": {
"mnsami/composer-custom-directory-installer": "1.1.*",
"getkirby/kirby": "^2.4"
},
"extra": {
"installer-paths": {
"./kirby": ["getkirby/kirby"],
"./kirby/toolkit": ["getkirby/toolkit"],
"./panel": ["getkirby/panel"]
}
}
}