Using something like a helper in model

Hi there!
How to start... I have three tables with, let's say, "comments", "pages" and "somethingelse". All have a colmn "slug", which is the ID in the URL to access them directly. For all this three tables I have a model and in all three models is the same method "createSlug()" which is doing exactly the same: creating a slug, check it against the database an increment a sufix to the next one or a free one between others.

PHP Code:

    public function crtSlug(string $name, $test = FALSE)
    {
        // Slug is made of the name/label 
        $slug = mb_url_title($name, '-', TRUE);

        // check usage
        $chk = $this->builder()
            ->select('slug')
            ->like('slug', $slug, 'after');

        $used = $chk->get()->getResult();

        // create an array with all used _n IDs
        $usedDB  = [];
        $i = '0';
        foreach ($used AS $str)
        {
            $sub = explode('_', $str->slug);

            if (empty($sub['1']))
                array_push($usedDB, '0');
            else
                array_push($usedDB, $sub['1']);

            $i++;
        }

        // create an array with the content lines
        $rowsCnt = range('0', $i+1);

        // compare both arrays and save the missing numbers
        $next = array_diff($rowsCnt, $usedDB);

        // create slug with the first number of a reseted array
        // and return it
        return mb_url_title($name.'_'.reset($next), '-', TRUE);

    } 


My first Idea was to create a helper, but as you see, the method uses the model and the helper has no access to the "builder()".
So my question is: how to create global method which are only works local, in the specific model?

Thanks!