I have heated water to 196 F in the matter of just a few hours.
Step 1: Materials Needed
1 Flexible reflector auto shade (the shinier, the better)
1 Large plastic bottle with a screwable mid-section (This is hard to find. I got mine at a dollar store and it contained a lot of small bungees. This is convenient, but not necessary. You can use a 1 liter soda bottle, a half gallon square water bottle or a large jerky container.)
Metal duct tape.
Regular Ducting Tape.
Self-stick Velcro strips.
Step 2: Notes on Bottle Types and Hazards
The photo shows the alternate outer bottle - a 1 liter PET soda bottle with the bottom cut off about 2 inches from the bottom and attached on just one side with duct tape. This allows you to hinge the bottom to load the black-painted HDPE bottle.