I have taken the expected dimensions from the web page that describes the project and confirmed the size of the Horn.

The numbers match those stated in the web sites drawing of the metal sheets. It was good though to walk through the basic geometry though.
Once I had the angles calculated I started to cut up some Amazon packaging.
I used a Protractor from a Geometry set I bought years ago as I thought it would be useful to have.
These were best endeavours as the angles were 22 and 17 but these were decimals  which a school protractor can resolve.
Not to worry this is good enough. I have now obtained a digital protractor which is two steel rules with a joint and digital display showing the angle between the two steel rules. I will need this way further down the line.

Talking Protractors I have now also have a device which gives a digital read out of tilt angle, this will be useful to see where the Horn is pointed at.

I've updated the 'ariel'. Copper rod can't be 'soldered'. I tried to drill a hole in the copper rods I have but that didn't work out. Amazon can supply copper tubes, who knew !

A 2 mm tube is just needs a bit if a crimp to connect it to the solder point of the N Type connector.

I have plans for supporting the Horn, and it continues the theme of using cardboard and re-purposing.

Construction has started. I don't have any more prepared sheets as I need to be quiet for a couple of weeks, but can get some raw sheets cut out.