I liked the approach of the cover made by beekeeper but I wanted to make a couple of changes, and I'm not a SketchUp person, so I remodelled in OpenSCAD. In my design the switch is on the front face, which means you can print without supports. At first I had the slot the same width as the connectors, but I managed to get a very small switch which allowed me to feed the mains cable through a hole and keep all the mains wiring hidden inside.The OpenSCAD model is parametric, you can easily adjust the overall width of the cover and the wall thickness. Other changes may need a bit more effort. As in the original design, there are raised ridges which clip into the slots in the casing. You may need to adjust the size and position of these, I don't know how standard these cases are.