WARNING!

If you use this box with high-power devices or an improper cable junction, the generated heat can melt the box and set the plastic on fire. Please be careful with what you do and seek professional advice if you have doubts. Also, be aware I don't know your local, regional or national regulations: in some cases, you might be violating the law.

Instructions

I needed to extend a power cord for an indoor appliance and I wanted a secure solution so I designed a small box to accommodate a 3-wire junction using a simple terminal lock, as depicted in the images, for 2mm wires (Aliexpress affiliate link). 

Each connector of the block is 6mm wide and I suggest you measure your blocks before printing as many variations exist.

The two holes are 8mm in diameter and I used two zip ties to make the assembly more secure. 

The cap is quite a snug fit.

Material

Proper electrical junction boxes are made out of fire-retardant plastics, which are not available for extrusion. You probably want to print this out of the material with the highest melting temperature, but if you believe this junction will ever get hotter than a couple of degrees than room temperature, I would advise AGAINST this model, or any other 3D printed model, and go for a properly designed and certified junction box, or a longer certified cable.

Customizations

If you need some variation on the theme, feel free to leave a comment with your terminal dimensions and I'll try to adapt the model.