Selly's Plastibond.
It's a plastic putty, and you mix it with a hardener (it comes with the main tube). Drys in roughly 20 minutes and is rock hard, meaning you can actually drill and screw into it and know it will hold.
The stuff is actually sold for puttying up metal and ceramic objects, such as tiles or fence posts.
And it costs only $7 AU. Well worth it, and I've used it all around the place knowing that the screws I put in it will hold the monitor (and tray).