John, I make test connectors similar to this for fuel injection harnesses, OBD2 testers, etc, etc, ….I use normal heat shrink tubing and clear silicone, ……put a dab around the header connector where the wires go in, slide the heat shrink up & over, wipe up any excess, wait about 15-20 minutes for the silicone to skin over, then shrink the tubing, when the silicone fully cures, it’s a tough, strong test plug that can be tugged on hundreds of times. …..sometimes I might use two sizes of heat shrink depending on the wire bundle …..one to surround the plug, and another that surrounds the wires neatly, but also bridges the larger one ….small tube of clear silicone is plenty cheap enough, and only cut a tiny hole in the tip so you can control ooze,….
Pat
