Rewrite rules - if file exists in different location

Hello,
Need a bit of help with a rewrite rule.
I've got the basics which I've developed from various bits and pieces over the years but facing a new problem which I can't quite work out.

RewriteEngine on
 
RewriteRule ^index\.php$ - [L]
 
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -f [OR]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
 
RewriteRule . /index.php [L]

Code (markup):

After the test that checks a file/directory exists I need to do a similar thing, but check a different directory.
In my head it would work something like this.

RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -f [OR]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -d
RewriteRule (.*) /dynamic-folder/$1 [L]

Code (markup):

But the part I can't work out is how to check the specified directory in the rewrite rule.
I wonder if it's something along these lines.

RewriteCond %{DOCUMENT_ROOT}/dynamic-folder/%{REQUEST_FILENAME} -f [OR]
RewriteCond %{DOCUMENT_ROOT}/dynamic-folder/%{REQUEST_FILENAME} -d
RewriteRule (.*) /dynamic-folder/$1 [L]

Code (markup):

So the final result would be something like this.

RewriteEngine on
 
RewriteRule ^index\.php$ - [L]
 
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -f [OR]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
 
RewriteCond %{DOCUMENT_ROOT}/dynamic-folder/%{REQUEST_FILENAME} -f [OR]
RewriteCond %{DOCUMENT_ROOT}/dynamic-folder/%{REQUEST_FILENAME} -d
RewriteRule (.*) /dynamic-folder/$1 [L]
 
RewriteRule . /index.php [L]

Code (markup):

Any help would be greatly appreciated.