what I use to protect my bike from stratchs on the top yoke are those little removable rubbers which go on the end on pencils.
The good thing about this:
If you have three keys - place two rubbers on the other two - when you use your disk lock, transfer the rubber across onto the ignition key - then when you return to your bike you have to remove the rubber from the disk lock key, this reminds you that you have your lock on.
Admit it we all have tried to drive off with the lock in place.