The PCB was designed to fit in several enclosures, so if you use it in a rough environment you can protect it easily.
It would fit perfectly tight in a Hammond 1590B, with a bit more room in a 1591C and with plenty of space in any of the bigger boxes like the 125B or the 1590BB.
The 1591B is too small for the PCB. This box is made of plastic and cutting the internal pillars the PCB will finally fit inside but I did not try it yet.
oh sorry, the small holes are for M3 screws (so they should be around 3.1mm holes) and the big ones anything between 5 and 8mm would do. Fortunately, there are not tight tolerances for the big holes.