When the temperature hit 100°F this June I knew I had to avoid lighting my propane stove during the day. I have a box-style cardboard solar oven which I've used for years for granola and beans and even a pie now and again. However, it's bulky to haul out just to heat soup for lunch. Research led me to favor building a solar funnel
like that designed by BYU's Professor Jones. They're efficient and a lot easier to build than a parabolic reflector. Mine took about two hours to build.
Now if I'd had a roll-up car sunshade
(see also wsalazar's Solar Cooker
) I would've used it and saved myself a few steps. Since I didn't, I fell back on the tried and true cardboard-and-foil approach, using things I had at home. As it turned out, the stiffness of the cardboard makes this cooker very easy to adjust and secure.