I upgraded my Prusa Bear heated bed to a 220V one and designed a protective cover for a SSR-40 DA to safeguard against electrical shock. The side hole is specifically for mounting it to 2020/2040 extrusions. You'll need two M4x8mm screws to secure both the relay and the cover.

There’s a long narrow hole between the two circular holes, designed to allow me to insert the wire without cutting it. Note that this requires support for printing.

Big thanks to Santiago Garcia for uploading his SSR model:
 SSR Model on GrabCAD