The Mirage was powered by the French Atar 9C engine.
The original version had a 2D converging nozzle
in that two large doors moved from fully open to a bit closed in order to maximize the thrust at various conditions and settings ( sort of like squeezing the end of a garden hose with your finger. This Mirage was delivered with such a nozzle.
Later on in its life it was converted to a full 360 actuating flap configuration
I double checked with Yoav to confirm that the model as depicted here had the 2D nozzle.
So, here are the parts and construction. Nothing complicated.
the dummy disc is held in place while super glue is applied around the cyliner to maintain a good round exhaust section.
Isaac