A foam box with a sheet of glass on top should make a pretty good solar collector. It is well insulated and the glass should stop any captured heat from escaping. If it's painted matt black inside it should work even better.
To have something to compare our results with I have a second identical foam box. I've placed a thermometer in each, but I've placed the digital readout next to the other thermometer so I can see them both at the same time when I take a photo. The box on the right has the digital temperature probe in a similar position to the other box's thermometer. The box on the right also has some black plastic in it and is covered by glass.
To make sure both thermometers were working and showing the same temperature I put them in the same box for a while.
Both thermometers showed 22 - 23 C in the same box after sitting in the sun for around 20 minutes.
It was 20.8 in the shade according to a third thermometer. I have a lot of thermometers.
This was the amazing result after 36 minutes!
+ 45 C in 36 minutes!
I knew there would be an effect but I had no idea it would be as big as this.
Did I mention it only took 36 minutes!
And 68 deg C! That's slow cooker temperature! Is that slow cooker temperature? Yes* it is! And on a 20 degree day! In only 36 minutes! I wish I had some aquaponics produce. I want to make some laksa!
* from Wikipedia RE: food safety ...
The UK Food Standards Agency publishes recommendations as part of its Hazard Analysis and Critical Control Points (HACCP) programme. The relevant guidelines at http://www.food.gov.uk/multimedia/pdfs/csctcooking.pdf state that:
"Cooking food until the CORE TEMPERATURE is 75 °C or above will ensure that harmful bacteria are destroyed.
However, lower cooking temperatures are acceptable provided that the CORE TEMPERATURE is maintained for a specified period of time as follows :
- 60 °C for a minimum of 45 minutes
- 65 °C for a minimum of 10 minutes
- 70 °C for a minimum of 2 minutes"
Previous guidance from a leaflet produced by the UK Department Of Health “Handling Cooked Meats Safely A Ten Point Plan” also allowed for:
- "75 °C for a minimum of 30 seconds
- 80 °C for a minimum of 6 seconds"