New ID Column Design

I need to periodically send a set of data to a vendor. They need to have a column what is an unique identifyer but they cannot handle 16 digit GUI.So I have this my_old_person table with person_GUI as uniqueidentifier. It got 200K rows in it. I am thinking creating a staging utility table:

Create table my_new_person
(person_GUI varchar(255) not null,<--this will contains the same GUI from the old
Person_ID nvarchar(20))<--send tis to vendor

At the beginning of my nightly ETL, I will run this to get the new data:

Insert into my_new_person
select a.person_GUI, xyz
from my_old_person a join my_new_person b
on a.person_GUI=b.person_GUI
where not exists (Select person_GUI from my_old_person)

How do I implement this person_ID to make it unique and populate correctly in above xyz?