If you can't get any just dremel a nice u shaped hole in the side of the the top of a normal 4mm connector and offer the tinned wire into the top of your groove as you solder it, might need heat to pursuade it in, works perfectly, holds the wire nicely as you finish the joint
Don't forget to put the heatshrink on the wire first and cut a little v shape into the heatshrink so it fits well, and make sure the heatshrink is far enough down the wire so the heat doesn't get to it as you do the joint