Taken from the cardboard model here: https://www.htpc-diy.com/2012/04/diy-flexible-fractal-window-hdtv.html
I thought a plastic (PLA) version might be a little more robust, and accurate since I don't do the cutting.
Put the two pieces together, you may need to ream out the hole in the top piece to make it fit. then I used contact cement to glue on aluminum foil ( but any spray glue or even glue stick should work), and cut along the edge with a sharp knife.
The Balun is a cheapie off of ebay,( 300-75 ohm) it was about $1. I secured it with a couple of M4 8mm screws. But really almost anything would do, as long as the balun connectors make good contact with the foil.
The second file is a hanger for the two sides. they go on with light pressure, and seem to stay on well.
I was pleasantly surprised that we pulled in many HD channels with it.