Simple, really - Just need to get clear flexible vinyl strain relief molded in sticks sized to fit a Hot Glue Gun. And a new, clean hot glue gun with the right temperature thermostat inside too, of course.
Use it freehand to get the splices insulated, then form a little dammed-off cavity mold around the connector and fill it up the rest of the way. Sculpt as necessary. The Clear is important, so you can see what you did wrong if it fails later.
That, or you mod a glue gun with a little hopper and a pusher plunger, and get the vinyl in the usual industrial molding press 1/16" pellets form.