Radar reflectors do work, but the device G described is an orthogonal reflector and not a Luneberg lens.
In a previous existence I ran a place that tested and measured radar cross section (RCS) and as a service to small boat owners conducted a whole series of measurements on various types of radar reflector. This was published in Practical Boat Owner magazine a few years ago, over about three issues.
Overall, a simple, adequately large, orthogonal corner reflector works as well as, and often better than, any of the expensive devices. The only merit that the luneberg lens posesses is that it gives an accurate radar cross section, making it useful as a calibration aid. Compared with a simple corner reflector it is heavy, expensive and gives virtually no real advantage in RCS.
Making a lightweight corner reflector is easy. Just get some 1" thick expanded polystyrene insulation sheet (styrofoam for our colonial cousins) and cut out three squares, as big as you can fit into the space you have available (big is good!). Cut slots from a corner to the centre of each square and very carefully glue them together so that the internal corners are exactly at 90 deg (this is important for best performance). When the glue has gone off, carefully glue aluminium cooking foil (aluminum for our friends over the pond) as smoothly as you can to the inside of every internal corner.
The result will be a very effective reflector that will be extremely light in weight. For best results, mount it so that one open face points upwards at normal flying attitude, rather than having it with one point up.
Take a look at pictures of the alloy corner reflectors in adverts to see how the thing should go together (a google search should hopefully show some pictures somewhere).