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.
Step 1: Materials and Tools
6' 1" of 18" heavy-duty aluminum foil;
Water-soluble glue (~3 oz. Elmer's Glue-All);
3 or more brass paper fasteners;
1 piece ~8" round aluminum sheeting (or a round of cardboard covered with foil);
1 bucket or planter;
2 medium-sized binder clips ;
~1 yard of 1" to 1-1/2" wide galvanized sheet roofing;
~10" square of 1/4" mesh hardware cloth;
1 1" machine screw with nut;
Framing square (handy but optional);
Tape measure or ruler or yardstick;
Sharp knife or box knife or drywall saw;
Something round for a template, or a drafting compass;
Brush or paper towel or rag;
Scissors or nail or icepick;
Pliers and screwdriver.
Step 2: Cut Cardboard
Figure out how big you want the hole at the bottom of your funnel. No opening at all (so your funnel is actually a cone) could make standing it upright in the bucket difficult, whereas an opening more than six or eight inches across means losing too much reflective area. I used a slightly less than six inch opening, which meant cutting a 12" diameter half-circle out of the cardboard.
Center your half-circle by finding the halfway point of one of the 4 foot sides of your cardboard. Draw the half-circle using a compass or a handy template like a five gallon bucket lid.
Carefully cut out the half-circle. I used a sharp, narrow-bladed pocket knife, but a box knife or drywall saw will also do the job. Avoid cutting yourself if possible, though sometimes I think a little blood adds soul to a project.
Step 3: Apply Foil
Dilute water-soluble glue is easy to apply and easy to clean up. It lasts pretty well too, assuming you don't leave your funnel out in all weather. I've tried spray auto trim adhesive and found it doesn't last nearly so well.
Mix 1 part glue with 1 part water or less, to make it easier to spread. I used about 3 ounces of Elmer's Glue-All, and perhaps a quarter cup of water. I applied it with a folded paper towel. A brush would be less messy; a rag would work as well. Apply the glue only on the strip of cardboard your current piece of foil will go on.
Using clean hands! apply the foil, shiny side up*, on the glued section. Avoid wrinkles. Avoid getting glue on the face of the foil.
Now remove the glue you got on the surface of the foil in spite of all your care by using a clean, damp paper towel or rag to wipe it away. Clean the area several times.
Trim the extra foil from the edges. If you trim carefully, the extra foil from the semicircle gap can be used to cover a round of scrap cardboard to fill the base of the funnel (see step 6).
*There is some experimental evidence that a matte aluminum surface actually works better. It is therefore possible that using the less shiny side of the foil would be preferable. I went with tradition this time.
Step 4: Shape the Funnel
Keep in mind that the point where all these creases should meet is not at the mid-point of the half-circle, but the still theoretical mid-point of the circle you are going to create when you join the sides. (See diagram). In my case this point was about three inches up from my original center point. But approximating this will work fine, so don't sweat it.
Gently persuade the cardboard into the shape of a funnel, overlapping the edges by an inch or two.
Poke three (or more) holes along this seam, using a nail or icepick or scissors blade or whatever's handy. Put a paper fastener (like a small brass cotter pin with a round head) through each hole and fold the tails outward to join the two sides. Using twine or thin wire such as twist-ties might work just as well.
Step 5: Fill the Gap in the Funnel Base
Use the base of your funnel as a template to draw a circle of the right size on the aluminum sheet, making sure to leave an inch or so of material around the edges of your circle.
With your tin snips, cut out a circle of aluminum roughly two inches wider in diameter than the funnel opening. This doesn't need to be a nice neat shape. Mine certainly wasn't.
From the edge of your aluminum, make a series of cuts just to the line of the inner circle. Think of this as though you are making petals of a flower with a rather large center. You may want to round off the corners of these petals so they don't stick you.
Bend the petals upwards at roughly a 60° angle, working methodically around the circle so that each overlaps the one before.
Push this down into the funnel. It should sit there quite happily on its own.
If you don't have aluminum sheeting handy, there's no reason why a round of cardboard covered with foil won't work just as well. Just make it big enough that it won't push through the opening. A lightweight aluminum pie plate might also do the trick with no work at all, if your opening is sized to fit it.
Step 6: Build Pot Stand
My stand is just a 1 1/2" strap of metal bolted together at a single point to make an 8" diameter circle. Over this goes a piece of 1/4" mesh hardware cloth folded to fit, trimmed so the edges of the hardware cloth don't poke holes in the foil of your funnel--or your fingers. This stand allows a lot of heat to be reflected up onto the bottom of your cooking pot and envelope.
If you can find an 8" to 9" diameter wire cake cooling rack, that will work very well also.
Step 7: Options: the Pot and Envelope
It is possible that the oven bag envelope would heat up quicker, and you may want to go that route. An ordinary supermarket type 2 vegetable bag will serve in a pinch, as long as you can keep it from touching your cooking vessel, the heat of which will promptly melt holes in it.
Step 8: Cooking: Setup and Adjustments
Place the solar funnel, narrow end down, inside your bucket and aim it at the sun. I find the "horns" of the funnel helpful here. Ideally from one perspective the sun should sit halfway between the horns, and when you move 90° around the funnel, the horns should point a few degrees higher (if it's morning) or lower (if afternoon) than the sun. That way you've got an hour or so before you need to readjust the aim.
As breezes are a regular feature of my local weather, I secure my funnel to the bucket. My planter had two rings, one on either side. I tied an approximately yard-long piece of string to each ring. On the other end of the string I tied a mid-sized binder clip. The binder clip clips to the top edge of the funnel, with the string caught so there is no slack. This holds the aim as well, and is easy to readjust when necessary.
Assuming you don't have handy rings, wrap a piece of wire around your bucket under the lip, with a loop on either side to which you can tie your string.
(This, by the way, is one advantage of using cardboard over a sunshade, which isn't stiff enough to clip securely into place.)