This design covers the entire bottom of the arduino 4 channel relay board so if you running 120v you won't shock yourself and it gives you 2 screw holes to mount this module.
The design takes 10-15m to print and uses 2000mm of plastic. The part is a snug fit so check your printers calibration is within around 1mm. Be careful removing the part from the build platform, the base is 2 layers thick but it has lots of surface area to stick.
Be careful with the 4 posts, they are thin and will snap off if you are applying too much pressure to get the board into the cover.