There's also a 2-part epoxy mix intended for repairing rubber / plastic parts, mainly for automotive use - engine mounts,accordion air hoses, dashboard parts etc. It sticks to just about anything, and sets to a hard rubber consistency. It is very tough, I used it a lot to repair the soles of my childrens' shoes when they were at an age where they would scuff holes in them in a matter of weeks. I have also used it for exactly the job you did - fixing proprietary plugs after cutting them open. I discard the cut-open plastic, and cut and fit a piece of heat-shrink to form a mold. I fill the mold with enough epoxy to cover the connections, then heat-shrink the rest to form a strain relief around the cable.
The brand I use is Pratley Wondafix Car: Pratley High Performance DIY Adhesives