I came across and article in an old electronics magazine on how to make a directional microphone with a cardboard horn. I decided to copy the horn part.
The horn consists of four triangle shaped pieced of cardboard cut from boxes. Each of the four pieces is 1 inch (25.4mm) at the small end, 12 inches (300mm) at the large end, and 24 inches (600mm) long. The line from the small to large end has a 1/2 inch (12.7 mm) ‘sag’ at the 12 inch point, and 1/4 inch (6.3mm) ‘sag’ at the 6 and 18 inch points.
I assembled it with short lengths of clear adhesive tape, then covered all the seams with the heavier 2 inch wide box tape. The curved sides help stiffen it, but it still wasn’t very stiff. I hot glued most of the seams on the inside, and that helped stiffen it a lot.
I still have to mount the microphone and connect it to a microphone preamp.