Somewhat unrelated to trikes, but applicable, I was doing a carb swap today and needed to shorten the throttle cables.
Old carb, left, new (to me) on right.
I only had to shorten the cable, and not the housing, but this works the same either way. As luck would have it, a nearby parts store had some cable stops. An assorted package of them was ~$3.70. I only needed the two 1/4" ones for this project.
The larger ones could possibly be used for brake cables. I had to turn down the 1/4 stops just a smidge, to fit the carb. I done this by putting them on the end of a long screw, fixed in a drill, and turned against a file clamped in a vise.
I cut the original cable ends off and removed any brackets or nuts from the adjusters that I wouldn't need. Then I positioned the cable stop where I needed it, placing the stop screw barely a thread in, just to keep the solder from coming out the bottom. I did clean the cables with brake parts cleaner and used soldering flux. Then I slowly put solder in, just enough to fill it. I didn't use anything fancy, just a convenience store butane torch to melt the solder.
I've got actual propane/MAP gas torches and soldering irons, but I just used something very much like this:
After removing the screw, which take a little effort, the entire cable stop is mostly filled with solder. A small void is there, but the cable is held securely.
The final step is filing or grinding away the last bit of cable. I used a Dremel with a grinding wheel, after sniping away the bulk of it with a pair of dykes.
Nothing more special than a drill used, along with some actual cable cutters (bicycle cables) for the first cut. The cut after the stop is soldered on is easily done with regular dykes. A hand file will take care of the much of the final prep, but a rotary tool with a grinding wheel speeds things up.
The larger cable stops would probably work for the brake cables on many trikes.