I found the answer myself - so here is the update to above problem:
Custom post type registration:
function create_post_type_mycustomname() {
$args = array(
\'capability_type\' => \'post\',
\'has_archive\' => \'mycustomname\',
\'rewrite\' => array(
\'slug\' => \'/mycustomname\',
\'feeds\' => false
)
);
register_post_type(\'ctp_mycustomname\', $args);
}
add_action(\'init\', \'create_post_type_mycustomname\');
Change the links:
function mycustomname_links($post_link, $post = 0) {
if($post->post_type === \'ctp_mycustomname\') {
return home_url(\'mycustomname/\' . $post->ID . \'/\');
}
else{
return $post_link;
}
}
add_filter(\'post_type_link\', \'mycustomname_links\', 1, 3);
Add the correct rewrites rules:
function mycustomname_rewrites_init(){
add_rewrite_rule(\'mycustomname/([0-9]+)?$\', \'index.php?post_type=ctp_mycustomname&p=$matches[1]\', \'top\');
}
add_action(\'init\', \'mycustomname_rewrites_init\');
Flush rewrite-rules in Wordpress backend afterwards and you are good to go!