The optimal solution is usually thus:
You move each of the cylindrical features into its nearest corner and mold it in flush with the side walls.
You extend all eight of these bosses (that's what those tall cylindrical features are called) flush with the parting line at the center. You make four on one side into hollow tubes where the tube goes all the way through the boss to the outside of the part and the tube has enough diameter to pass a screw without interference.
The bosses and the side walls of the part are then relieved by tapering them by about 1 to 1.5 degrees, so the molded part will free itself more easily from the mold cavity.
On the mating half of the box, you put small shallow holes into the ends of the bosses that are the right diameter to engage the screw threads.
To assemble the box, you fit the two halves together and run a long screw into each of the thru-holes on one half so the threads engage the holes in the tips of the bosses on the other side, and tighten the screws.
Note also that it is standard design practice to pilot the mating surfaces together so that they interlock when pressed together. This will automatically bring the screw features into proper alignment during assembly, so the screw point will "find" the hole it is supposed to screw down into.
To save a few pennies on each assembly, you can step the diameters of the thru-holes so as to pass the head of a screw and then stop the screw head near the bottom of the thru-hole with a short section of smaller diameter hole. This lets you use a shorter (cheaper) screw.