Holes for the switch, screws and mains cord were simply melted into the plastic box using a previously discarded soldering iron tip. If you use this method, do it outdoors because the fumes may be noxious.
As with all such projects you should use common sense, remembering that soldering irons can burn and mains voltages can kill. The complete project should be finished so that it is impossible for any person (especially a child) to come into contact with any part that can carry a high voltage. If the unit is constructed as suggested, it will still be possible for someone to draw a small spark from the charged plate and get a nasty shock, albeit indirectly. This presents a similar risk to that of a car ignition circuit. In other words, the risk of injury or death is extremely low unless the shock recipient has a heart problem.
Eye protection and a face mask should be worn while using the flock applicator because the tiny fibres can cause bad eye and lung irritation.
The flock applicator is very easy to use and the results can be amusing, surprising and useful. Apply a suitable glue, paint, varnish or resin to the surface of the object. The adhesive should preferably be of a similar colour to the flock. Sprinkle a little flock onto the lid, hold the object above the flock and press the switch. Bring the object lower and the fibres will leap up and embed themselves, end-on, into the glue. The effect is more powerful if the object is grounded but, in practice, the presence of your hand is usually sufficient.
The flock can be ejected in a powerful spray so you should use it in a place where the resultant mess is easy to clean up and won't cause problems. Do wear eye protection, a breathing mask and clothing that can be discarded. Although flock is not quite as bad to work with as glassfibre, it can be unpleasant.
If humidity is high, the charge is quickly lost and results can be very poor. The flock fibres need to be completely dry - store them with a dessicant and heat them up if necessary to drive out moisture. For best results, sprinkle only a very small amount of flock on the lid and replenish as necessary.