The best radar reflectors are the corner cube variety (intersecting circles of reflective material) but to work they have to be oriented in the "Catch rain" position and when you see them on boats, very few are oriented correctly. If oriented correctly and weighted so they are always in this orientation no matter how much you heel they are very effective. They are difficult to mount properly. So, I decided to develop an inflatable version like a beach ball only to find that a brit company already had the patent and made them.
NOW, they are available here:
Echomax EMA031 Inflatable Radar Reflector
KInda pricey but a very good idea. I'm gonna get on now that I can.