This sort of housing assumes the pcb will mount perpendicular to the cover. That is, the card is just as wide or long as the inner dimension, and slides in between the ribs around the edge. These ribs, and their spacing, act as edge guides and supports. This means, of course, that your pcbs will be pretty small.
Alternatively, you can use a larger board which fits parallel to the cover. In this case, you drill holes in the back face, then mount standoffs which then connect to mounting holes in the pcb. If you do this, it's important to place elastic washers between the standoff and the back face to form a water-tight gasket for the screw hole. Generally, a metal washer between the standoff and the elastic washer is a good idea to spread the mechanical load on the gasket. Alternatively, if the back wall is thick enough, you can drill and tap for screws which will engage the standoffs.