function presentation($post_id){
 if($post_id){
   $presentation = new Presentation($post_id); // to populate
  }else{
   $presentation = current_presentation(); // switch to create
}

function current_presentation(){
  if( xxx_type_is_xxx_page() ){
    return new Presentation();
   }elseif( yyy_type_is_yyy_page() ){
    return new Presentation();
   }elseif(){
    //and so on...
   }else{
    return false; // do not create if not on the above post type creation page
  }
}