Edit Default Single Template with Child Theme

Hi,

I want to make changes to the general default single post template via php
As I’ve already searched through all of the documentation on how to work with child themes, my final understanding seems to be that only the templates 1-13 can be edited with a child theme but not the default post template?

So then I wanted to create a solution that is most likely update-proof and requires little effort to update.

I’ve created a hook: <?php do_action(‘after_single_post_footer’); ?>
In the /wp-content/plugins/td-standard-pack/Newspaper/loop-single.php:67
and then call the hook from my /wp-content/themes/newspaper-child/functions.php

add_action(‘after_single_post_footer’,’add_read_further’);
function after_single_post_footer () {
//some function
}

However this will not work. The only way I seem to be able to add actual content is when hardcoding the HTML into the loop-single.php, which is what I wanted to prevent.

Just in case, I’ve tried creating a page with the Cloud Templates and also was thinking about the API, but then again you seem only be able to edit the templates 1-13 but not the default one.
We have just over 2000 posts that are set to default template manually, so adding a new one is out of the question, as I wont manually change all of them.

Do you have any ideas on how I might be able to change the default post template via php such that I have a theme update proof solution?

Sure, I’ve just sent an email