In case anyone comes across this thread in the future, I found the threaded Sunlite cables at the LBS for under $5. The threaded part of the ferrule does fit through the wider part of the keyhole opening in the trigger shifter, but not through the narrower slotted part. I suppose just putting the ferrule in the wider opening might suffice, but it seems it might at some point work its way out and increase the cable travel if it rested outside the trigger (e.g. against the slotted region). So, per comments above, I briefly (seriously, about half a minute worth of effort) filed the threads furthest from the tip just enough that the threaded part can be set up into the notch. Works perfectly, and it was a very cheap solution.