Thank you for the great link, well worth taking the time to read if you paddle in waters that are shared with commercial traffic or larger power boats.
A few years back I had brought a Radar reflector back from a boating trade show. This reflector was supposed to be the answer to making kayaks show up on Radar. Before promoting the reflector we contacted the Coast Guard about working with us to do some field testing. The Coast Guard assigned an Auxilliary cutter to take us out to work with our volunteer paddler.
The new product was a flop - we were never able to make it show on the Radar we were using. As long as we were out we took the time to test other commercially available reflectors. Like the study you have linked us to the best reflector was the one with the most angles with height above the water being the greatest influence.
Even the best of the reflectors we tested gave a poor return on calm water when we went to ranges in the 3- 5 mile radius and beyond (this would be a typical range for a commercial boat navigating by Radar in limited visibility). The Radar we were using did give a good return off a single kayak when used in the 1/4 mile range.
On another occation we were preparing to make a late night crossing to a nearby island. A group of us were waiting just off the beach for a ferry to leave the dock so as to not be in his way while he was maneuvering near the dock. Just before leaving the dock the ferry put its searchlight directly on us - my guess is he had made a 1/4 mile sweep with his Radar before getting under way and had detected us.
Sea Kayaker Magazine