A radar reflector is mandatory for balloonists in the USA. It is a requirement from the Federal Aviation Authority (FAA) for high flying payloads to be visible to aircraft radar at high altitudes. This helps avoid collisions even in conditions of poor visibility.
Radar reflectors can be bought commercially or built yourself. We recommend doing it yourself because it is so easy! All you need is some cardboard as a support surface and aluminum foil as a minimum.
When building your own, the biggest requirement is that the radar reflector is visible from all angle--ie, it is more than just a flat surface. Any shape (cubular, spherical) will work. You can attach it to your payload train by cutting a hole through the center and slipping it onto the rope. You can use tape (or other knotting techniques) to keep it in place.