We are building a set of shelves to cover up a sewer pipe in my basement. The shelves will have a false back that covers the pipe then have usable shelves in front of it. The only problem is that there is no way to build this without covering up the existing light switches right next to the pipe. Our plan is to extend the wires into a remodelers box in the side of the shelving unit so it sits behind the false back. That will make the existing box a junction box. I realize that junction boxes need a cover to meet code. On a metal box, I would get one of those covers with the hole for the wire so it can come straight out and that wire gizzy that goes in the hole to clamp the wires tight (sorry, I can't recall what they are called right now, please edit to the right term). Do they make those for plastic boxes? Or what are my options?