Placed on the back of the solar panel this box gives you somewhere to connect your external wiring to each panel and add a diode etc as required. I am currently experimenting with making my own solar panels as cheaply as possible and the commercially available terminal boxes are very expensive for what they are.I have placed a number of hole options in this box and will revise the number and position of these as my solar panel project proceeds. Uses 2 x 12mm long M4 machine screws for terminals and 1 x 20mm long M4 machine screw to hold the lid on.Print it out. Clear out the 4mm holes with a 4mm drill bit. Fit three x M4 nuts in the central nut chamber (sounds like my workshop). Fit 2 x 12mm long M4 machine screws and washers on the two outside holes for the terminal screws. Fit the lid with a 20mm long M4 machine screw to hold it on.I may change these machine screw sizes when it finishes printing, and I will probably make a larger version of this once I have worked out hw to make it all work.