My kettle has over-boiled on several occasions since I made it.
Yesterday it over-boiled by 10:30am. The min/max temps were 17C(62F) and 29C(84F). Sunrise about 6:30 am.
Beware you can easily scald yourself when the water boils.
For the reflector I used a 25 liter paint bucket covered with Aluminium Foil. The inside bottle is a 750ml glass wine bottle.