Just use the same
brake housing you would use with your regular brakes. The interrupter levers just pass the inner wire through as if they were a section of the housing when you are not using them, they do not require special housing. The cable ends, all of them not just the ones which interface with the interrupter levers, need to be properly prepared.
https://www.sheldonbrown.com/cables.html Scroll down to the "Cutting Housing and Preparing the Ends" section.