, I used a dark metal roaster in a large cardboard box lined with aluminium foil, shiny side towards the roaster to reflect heat towards all sides of the roaster. I put a large piece of glass from an old window, that I cleaned with Windex before hand to trap the heat inside the box. The temperature inside the box went to 300 F. I started at 11:00am, put the roaster in at 11:30 and left it there for 3 hours without opening it. I cooked a 3 1/2 lb sirloin tip piece of beef. The outside was well cooked but lots of color at the center. Very juicy and tender and tasty if you like meat that way.
Takes a lot of time, a fair bit of prep and of course lots of clear blue sky. I prefer cooking over an open fire, much faster and can be done regardless of the weather.