You need to insert in the mid point of the elbow (normally 29) and to use pipe element not rigid element.
Many times the exact location of the dummy insert is not required but if you need to maintain the modeling technique as you describe, note that isogen is a piping software where doubling the elements in the same space is interpreted as an error.
You can use a copy of your normal file just for the isometrics purpose where you replace the rigid with this pipe element.
Regards,