Abstract custom Composer installer - https://getcomposer.org/doc/articles/custom-installers.md

AbstractInstaller.php

<?php

namespace My\Composer\Installer;

use Composer\Package\PackageInterface;
use Composer\Installer\LibraryInstaller;
use My\Composer\Installer\Exception\InvalidPackageName;

abstract class AbstractInstaller extends LibraryInstaller
{
    /**
     * @var string vendor/foo-
     */
    protected $prefix;
    
    /**
     * @var string path/to/my/install/dir/
     */
    protected $installPath;
    
    /**
     * {@inheritDoc}
     * @throws InvalidPackageName
     */
    public function getPackageBasePath(PackageInterface $package)
    {
        $prefixLength = strlen($this->prefix);
        $packageName  = $package->getPrettyName();
        
        if ($this->prefix !== substr($packageName, 0, $prefixLength)) {
            throw new InvalidPackageName($packageName, $this->prefix);
        }
        
        return $this->installPath . substr($packageName, $prefixLength);
    }
}

composer.json

{
    "name": "vendor/foo-bar",
    "type": "composer-plugin",
    "license": "MIT",
    "autoload": {
        "psr-4": {"My\\Composer": "src/"}
    },
    "extra": {
        "class": [
            "My\\Composer\\Installer\\FooInstaller",
            "My\\Composer\\Installer\\BarInstaller"
        ]
    },
    "require": {
        "composer-plugin-api": "~1.0"
    }
}