There are quite a few results for add trailing slash .htaccess on Google, but all examples I found require the use of your domain name, as in this example:
R
This should work:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !example.php
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ http://%{HTTP_HOST}/$1/ [L,R=301]
You don’t need to specify the domain, you can simply use an absolute URL path:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*[^/])$ /$1/ [L,R=301]
That does also make a check for the URL scheme obsolete.