I'm using solar vacuum tubes from Clean Republic. Like a high-quality thermos, they're double-walled containers with a vacuum in between to minimize heat loss. The dark outer coating of the tubes absorbs infrared energy from the sun, while the highly reflective inner layers of steel, aluminum, and copper trap that energy inside.
In ideal conditions, these solar vacuum tubes are said to be able to heat water to boiling in one hour. Under typical conditions they can't necessarily heat water to boiling, but fortunately the ideal water temperature for making matcha is sub-boiling, around 75 degrees Celsius. To monitor the water temperature inside the tubes, I am developing an Arduino-based thermometer using a waterproofed version of the 1-wire DS18B20 digital temperature sensor.