DBMS_DATAPUMP (Multiple Schemas) - need some suggestions for the parameter

I'm trying to export multiple schemas using DBMS_DATAPUMP and have run into a bit of a conundrum.

This line only accepts on schema at a time: So as long as my loop only contains one schema, it works

            DBMS_DATAPUMP.metadata_filter (
                handle   => h1,
                name     => 'SCHEMA_EXPR',
                VALUE    => 'IN(''' || rec.username || ''')');

This line "appears" to work better but actually doesn't import anything unless there's only one schema in the loop.

        dbms_datapump.metadata_filter(h1, 'SCHEMA_LIST', ''''||rec.username||'''');

So, how can I add multiple schemas via Loop ? I've been putzing with this for several hours now and hitting a brick wall on this.

There will be about 10 schemas and I want the loop to be able to be dynamic if possible. The export is coming from a source code database so the data size is going to be small and will be used by developers running docker images.

I've got it working great for 1 schema - but I'd like to build it where it can handle doing all the schemas at once.

Chris