Quick summary. After experimenting with preliminary designs for both box ovens and parabolic reflectors, I am putting the parabolic design on the back burner due to its shortcomings (at least in my mind). They are:
Lots more reflector pieces to construct and put together.
A top heavy design that catches the wind
Focal point means a short cooking time or a complex tracking device
Heat radiates away from the pot too quickly
Anyway, the following are some data points.
We have some mirrors we would like to use so our first design attempt was use quadrilaterals. To get the parabolic curvature, these were not rectangles - nearly rectangles, but slightly off. That in itself was a problem - for the nominally 6 inch squares, the sides differed by 1/16 to 1/8 inches and the vertices were slightly more or less than 90 degrees. Then attempting to glue them or otherwise connect them was problematic.
If I were going to do this in a production mode, I might take a sheet of plywood and drill holes on 4-6 inch centers. And in those holes would put dowels at the proper height for the parabolic surface. The the cut pieces could be cut to fit and laid down so the 4 corners rested on 4 upright dowels. Connecting the mirrors together could be done with epoxy (though I am not sure of how well epoxy does being exposed to sunlight).
But there are a lot of mirrored pieces to be cut and glued -- very labor intensive, and the mirror assembly would need to have some sort of reinforcement structure.
One realization we had during this exercise was realizing the center of the parabola would not be getting sun since the cooking pot would be blocking it -- so may as well not put curved, reflective surface there.
The next variation on the parabola was to try "spokes" which consisted of different size trapezoids such that when the spokes are connected on either side there are roughly concentric circles reflecting to the focal point. The bottom is left empty of reflectors since the pot will cover that.
In terms of shape and work to assemble, this seemed preferable to the near-squares. To check performance, we build one of these parabolic reflectors using mylar from a camping "space-blanket". The challenge was to glue the mylar to the cardboard. On half we tried rubber cement and the other half tacky glue. The tacky glue did not smooth out well, leaving trails where it was applied (left photo). The rubber cement secured the mylar fairly smooth, but when exposed to the sun during operation, the rubber cement also wrinkled (center photo). In our trial run we had a pot at the focal point and an oven thermometer. The thermometer topped out at 205 (F). (right photo) To replace the mylar with mirrors and build a support structure to keep the assembly from toppling in the wind -- that would be more work.
Copyright 2009, Twin Springs Homestead Research Institute