hello,
intresting idea. firstly just to let you know Im an architect and I work in africa. I have used some solar oven and incorperated some into buildings. the collector is very important and so is its orentation. generally they are used for slow cooking e.g. stew and bread etc. they can be used to heat to extreamly high temps i know from Photovoltics and solar water heating applications.
the inventor in me recommends getting a reflectetive emergency blacket type material and then having a lightweight frame (e.i sticks/tent poles that can be poked into ground) to apply the reflector to, putting the collector in the center. then just waiting under the shade of a tree sleeping until its ready...however i am still trying to find a way to make a flying bicycle so im a optumist. howver the pesimists says
...+1 and you could eat them while riding to save even more time.
lol!!!
good luck, trial and error..trail and error..