This is a small switch box that fits perfectly on 20x40mm extrusion. I use this switch to turn LED lights on and off to light my print surface.

Switch: This isnt the exact switch that I used but this switch will work. You just need a switch that will fit in a hole 19x12.5mm. https://www.amazon.com/gp/product/B00VG9GYZ4/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A1SHAKOJOZ5BS5.