CDs don't have enough metal in them to reflect a
radar signal IIRC. Also, stacking them, means that the
radar is reflecting off the edge of the aluminum layer in the CD, which is only a few thousanths of an inch thick...not much of a
radar return off of that in any case.
The Davis Echomasters are among the better and among the less expensive
radar reflectors according to all the tests I've seen. I use one on my boat. They can be assembled around the backstay and hoisted along the backstay if you want, raised on a halyard, or mounted on a post. The Deluxe is slightly larger than the standard and comes with a halyard mounting kit, which is not included in the standard package.
If you use a halyard setup, you have to have two blocks, separated by about a foot, with one centered in the spreader and the other near one of the ends of the spreader.
This way, the reflector won't chafe the line that it is hoisted up with. I also added a "tail" to the
radar reflector, to help prevent it from twisting in high winds...acts as wind vane and steadies the
radar reflector. I have mine rigged to the upper spreader, and the flag halyards run to the lower spreaders.