A suggestion (although it may not be a great idea) - if you are using plastic pipe for your dust extraction, you may be able to find a cap that has slots or holes. These are used in drainage systems to prevent small creatures from entering the drainage system and finding their way back to a source (eg, back to the sump inside your house). The only problem with this solution is that the openings might be so small that it compromises the suction in your extraction system.
Hardware cloth is another option. I buy it by the roll to make protective cylinders for my wife to use to keep rabbits and other small animals from eating her plants. Its easy to cut and bend, so you can easily make a cap that just slips over the opening in your extraction system. It comes in various mesh sizes - 1/2" is fairly common, but there are others.
But the best solution might be chicken wire - same idea as hardware cloth, but with a much larger mesh that allows dust and shavings to be drawn into the extraction system, but blocks flying sandpaper disks. The only requirement is that the mesh must be smaller than the diameter of the disk.