Just because an object is shiny, or shiny with a hole in it, does not mean it would make an effective signal.
If you look at a mil-spec signal mirror you will see two sheets of polarized glass laminated together with a mesh-screen over the hole in the middle - in some cases the mesh has cross-hairs on it. Putting it to your eye, you will see a "bright spot" superimposed on the shaded circle - sorta like a "red dot" aiming sight on a firearm. That bright spot is where the 'flash' will be projected.
A search aircraft often will fly at 135 knots or faster. To signal the pilot, you need to flash the pilot of the aircraft. To do this, you need to be able to aim the bright spot right at the aircraft, and use it as an aimpoint to track the progress of the aircraft across the sky so the pilot notices a long constant flash.
This is going to be somewhat challenging with a dive knife, a mask lens, or a CD.
Not to say it can't be done, but remember this: from any altitude at all the entire surface of the sea sparkles on a sunny day with hundreds of "flashes" from sunlight reflecting off the water.
For a pilot to discern your signal flash from a natural flash off the water, your flash needs to "stick with her/him" long enough to separate it from the natural background phenomena.
Do yourself a favor and get a signal mirror. For grins and giggles, sit out on your back porch with a beer someday, and practice aiming the flash onto the tops of trees or some natural topography. It isn't rocket science, but it takes some practice to aim the spot exactly where you want it to go.
FWIW. YMMV.
