Have you tried running:

chgrp myuser /media/store/myuser && chmod g+s /media/store/myuser

This will make it so any item in the folder or anything put in there in the future is given the owner of myuser.

This should also work to:

chown myuser:myuser -R /media/store/myuser

Is this what you're trying to do?

Have you tried running:

chgrp myuser /media/store/myuser && chmod g+s /media/store/myuser

This will make it so any item in the folder or anything put in there in the future is given the owner of myuser.

Is this what you're trying to do?