Where can I find a custom
"luggage blind" OR "security shade"
for an Audi A6?

It is for the back seat of the Sedan,
(to hide things when the rear seat
backs are folded down thereby extending
the trunk), not for the Avant wagon.

I wonder if you've considered adding dark tint to the rear window.
That could be another solution.

How do I cancel the question?

My own answer: "Use a blanket" is sufficient.
I actually did a custom made one from the fabric store.