https://techcrunch.com/2017/11/08/researchers-discover-aluminum-foil-actually-does-improve-your-wireless-speed/

Researchers at Dartmouth University have found that a 3D printed shape covered in aluminum foil can improve wireless range and increase Wi-Fi security. The project, which appeared on Eurekalert, involves placing a reflector on and around a Wi-Fi router’s antennae to shape the beam, increasing range and preventing it from passing through to unwanted spaces.

“With a simple investment of about $35 and specifying coverage requirements, a wireless reflector can be custom-built to outperform antennae that cost thousands of dollars,” said Xia Zhou, a Dartmouth assistant professor.