Interesting. I didn't know about this.

What would be a good material to use to make the wire resistor?

I currently use long soft potentiometers for building controllers but they are expensive and could possibly be more expressive if I could make my own.

Doepfer’s commercial efforts at a reproduction substitutes a ribbon controller for the resistive wire. However it would appear he spent some time experimenting with this as there are notes on his page regarding it. He suggests a material called “constantan” to be wrapped around a nylon guitar string, then carefully polished to make a smooth enough surface. https://doepfer.de/traut/traut_e.htm#2.1.%20The%20Control%20...

I use the same type of soft potentiometer they use, and I find them kind of expensive. That constantan wire sounds somewhat promising but then all that custom work of winding onto a string and polishing sounds difficult or costly. I wonder if there is some sort of fishing line or something that would have useful electrical resistance.

Also I am not sure just how much of an influence it would have on tuning stability but it looks to have been chosen because of its temperature coefficient.