Add additional custom fields to idea portal "thank you" email templates

In the custom email templates for Idea portals, $Idea_Ref, $Idea_Name, and $Idea_Description are the only available fields we can use to customize the email. Our org. has developed a few custom fields to better capture information from our portal users, and it would be great if we could add additional custom fields to the email templates to make the "Thank you" email better align with the way users are entering data.