A combination of custom fields and the Single Post template should work for you here. References:
https://codex.www.ads-software.com/Using_Custom_Fields
https://codex.www.ads-software.com/Stepping_Into_Templates#Special_Template_Files
A few plugins that may be of help here can be found:
https://codex.www.ads-software.com/Plugins/Meta
One to look at is Get Custom Field Values:
https://www.ads-software.com/extend/plugins/get-custom-field-values
There is also a template tag for displaying custom field keys and values in a post:
https://codex.www.ads-software.com/Template_Tags/the_meta
It may be a bit underpowered for what you want though, as there’s no way to control the outout. A simple bit of code to accomplish a basic dump of a single custom field value is:
<?php echo get_post_meta($post->ID, 'html', true); ?>
That would work within The Loop (placed in your template) for a custom field with a key of ‘html’. It displays nothing if no ‘html’ custom field key exists with the post.
