Small pocket sized reflectors have been around for many years,
such as a solar cigarette lighter. These as well as other dished reflectors
gather the suns rays and focus them to a central point in front of the dish when
it is pointed towards the sun. The heat can be even hotter than that with a
burning (magnifying) glass, and some folks use this type of a reflector as a
solar stove with a pot hanging at the focal point. You may even have a makeup
mirror at home that could be used. these are usually the magnifying types.
To improvise a reflector you could take a shallow bowl and cover
it with aluminum foil, shiny side up and by holding this to the sun find the hot
focal point. You may have to try a few different bowls to find one whose focus
is neither too close or too far away. Of course improvised reflectors are not as
good as pre-made ones. Examples of where to find pre-made dishes that could work
are those small dish network antennas on many a roof, or the stainless steel
mixing bowls that are sold everywhere. Just remember: dish shaped and shiny.