How to create custom file dropdown and dialog

i am trying to create a custom dropdown for files in extending the core lists of dropdowns and dialogs. but i am somewhat stuck to get it work. i can get the dropdown to work but i dont know how to create the dialog properly with fields and link that to a model and the dropdown entry.

<?php
use Kirby\Cms\Find;
Kirby::plugin('XXX/plugin', [
    'areas' => [
        'site' => function ($kirby) {
            return [
                'dialogs' => [
                    'colspan' => [
                        'load' => function (string $id, string $filename) {
                            $file = Find::file($id, $filename);
                            return [
                                'component' => 'k-form-dialog',
                                'props' => [
                                    // field definition for the form dialog
                                    'fields' => [
                                        // TODO:
                                    ],
                                    // the prefilled model data
                                    'value' => [
                                        'colspan' => $file->colspan()->value(),
                                    ],
                                ]
                            ];
                        },
                        // https://getkirby.com/docs/reference/plugins/extensions/panel-dialogs#dialog-definition__submit-callback
                        'submit' => function (string $id) {
                            return true;
                        }
                    ]
                ],
                'dropdowns' => [
                    'page.file' => function (string $id, string $filename) {
                        $file = Find::file($id, $filename);
                        $dropdown = $file->panel()->dropdown();
                        $dropdown[] = '-';
                        $dropdown[] = [
                            'icon'   => 'grid-top',
                            'text'   => 'Spaltenbreite',
                            'dialog' => 'colspan'
                        ];
                        return $dropdown;
                    }
                ]
            ];
        },
    ]
]);

anyone has success doing something like that?

the Move Pages | Kirby CMS by @clicktonext seems to be able to create custom dropdowns. i will take a look at how they did it.