Trying to make a video balun to convert 75ohm unbalanced to 100 ohm UTP balanced is hard, when there is no design available and it is clouded is secrecy.
There is a patent for them out there, google patents has a copy, but from what I can see, there is no way it could work on composite video without DC restoration on the receiving side.
I decided to make a simple voltage balun, using ferrite rings from a old motherboard.
The result was a fail, the picture has poor colouring and the image was tearing.
My next design will be a current balun.