Hi @CCPECanvas,

I think a separate Canvas course shell is going to be the only way to accomplish this.  I don't know of any way to change access for just a single assignment in a course.

Just in case anyone else finds this thread, I'll also say that when doing a Turnitin assignment like this, you need to be very careful to turn indexing off, otherwise the submissions the students make to this assignment will get added to Turnitin's database, and their 'real" assignment submissions will be flagged as 100% plagiarized.

-Chris