With
F4 Simple Checkout Fields for WooCommerce you can simply add new fields to the WooCommerce checkout. There is no UI to manage the fields, they only can be added with a simple PHP method. That ensures that the plugin is lightweight and easy to handle, even though you need simple PHP knowledge and access to the file system to add the code (preferred your WordPress theme).
Usage
If you first install this plugin, it will do nothing. But it provides a method to add as many custom fields to your checkout as you need. Here's a sample how you could add a simple text field to the billing and shipping address:
add_action('init', function() {
F4\WCSCF\Core\Helpers::register_field(array(
'name' => 'demo-text',
'type' => 'text',
'label' => 'Text Field'
));
});