I ordered some of the connectors @
gugie recommended, haven't got them yet, so have not yet tried the following. But ..
I recommend the old wire crimpers made for Bell Systems. You can find them on eBay for $10-$20 or more. There are two versions, one with two small pair of dies in the jaws, which are useful for making Sturmey Archer gear cables, putting end caps on cables and, I strongly suspect, these wire connectors.
The other crimper has only one pair of dies, which I use for putting a spoke nipple onto a cable to make a brake cable end, so strongly I've not seen one fail.