Hi Alm
Thank you for your quick replay. I have seen those two posts already as well as Robrenz's videos on his measurement box which was very helpful. And the first post together with the Keithley Guide pointed me towards PTFE / Low Noise carbon lubricated cables.
Yes, the PL75 variant is a 3LUR TRB connector which should fit onto the Keithley 23x series.
If you build your own, you should use the 236 to characterize the cables. Force voltage and measure current with the cable connected and the termination open. Make sure to strap the LO to chassis at the instrument rear so that the shield of the 236 is at LO.
I have seen in person a DIY cable, built at a university, have horrible leakage from HI to LO. Probably dirt/grime/fingerprint on the insulator between HI and SHIELD at the connector. The cure was to cut off the connector and re-solder a new connector. The technician was advised to wear gloves and make sure everything was clean before and wile soldering the new connector. The new cable was leak-free.
I have created a demo for measuring insulation resistance of a piece of solid hookup wire with a SMU where the resistance is in the neighborhood of 25 T ohms. If you place a grimy fingerprint on the piece of wire, the measurement is destroyed. Wipe off the fingerprint and the measurement is restored.
This is one reason that I use authentic Keithley cables. They just work. They also have a conductive lubricant to reduce triboelectric effects.