I have Sierra disc calipers but not the original cable to cut down. Is there anything which is closer to the right size I can use instead?

IIRC I used one from an Austin Maxi.
Oddball I know but if it works it works....
HTH.
Cheers, Pewe10

I used Austin allegro for drums. No cutting needed.

If you use Austin Maxi of Allegro cables you will need 2