Do you have a 4-40 bolt, cordless drill, and belt sander? If you do, tighten the nut on the bolt. Chuck the assembly up in your drill. Run the drill in the opposite direction of the belt travel on the sander. This will evenly turn down the outside diameter of the nut. It may end up round- but as long as you don't take too much off it should be able to press-fit into the hole. Tighten it down, and put a small dab of crazy glue on the end to lock it in place.
I wouldn't drill the hole bigger- it might weaken the shoulder in the plastic casting.
You also might be able to find a blind nut in the hardware store- basically a tube nut with a flange. Cut or file off the flange and you will have basically what I described making in the first paragraph.