I would probably just buy three 4m extension cables and put them all together. Your cost would be under $500, and it is probably much easier than building a custom cable. Don't forget to include the cost of your time when you calculate the cost of a custom cable.
You might want to double check to see if a 35 foot cable exceeds the length limit for the signals you will be transmitting.
Bruce
Bruce Ammons
Ammons Engineering