There are lots of methods for PCB mounting in enclosures (casing) like snap fitting the PCB or mounting it using mounting holes (Boss) or using the standoff. Boss are a plastic feature for mechanical fastners.
For evive's PCB, we have used 4 mounting holes. The PCB is placed between the top and bottom plastic parts. The sandwiched PCB and casing are hold together using 4 bosses.
We have used self-threading screws of 2mm diameter as commonly used in electronics device enclosures. For designing the Boss, we have used this guide (or as per design catalogue of self-threading screw manufacturer)