Making straddle cables: I had to do that for my Herse tandem. Fortunately, I was able to use the old ends of the Herse straddle cables.
Herse did it like this: Machined the bell-shaped ends from brass. The thicker end is drilled out, so there is room for the solder. (Imagine a more or less constant wall thickness for the entire "bell.") Then inserted the cable, pulled it through, splayed it so it couldn't go out again. Pulled it back into the hole. Then soldered. (I didn't have to do step 1.)
When soldering, don't use a torch, just a strong electric soldering iron (50W or so). The torch burnt my ends, rather than provide a clean soldered joint. Use lots of flux and silver solder (not electronics solder). Model shops have that kind of stuff - many expensive model railroads are silver-soldered from brass.
With this technique, you should be able to make almost any straddle cable.
Jan Heine, Seattle