I fiddled around with parabolic dishes and fresnel lenses but in terms of cooking, I couldn't do much more than fry an egg or boil a couple cups of water. So I decided to take a shot at a more efficient way of solar cooking reasonable amounts of food like cakes and poultry. To do so, I chose a box design with reflectors placed all around it to gather more light and energy into the cooking area. It was a relatively easy build and it cost me no more than 30$ and I was thrilled by the results it gave me!

Step 1: Materials

A large mojority of the materials I used were gathered from friends who had them lying around or who wanted to dispose of them.
What I used:
Tools

A circular saw and its guiding rail
A glass cutter
A jigsaw (with metal cutting blade)
A compact drill
A grinder

Materials

8 mm thick plywood
18 mm thick plywood
glass pane from a picture frame
Glass or arcylic or stainless steel mirror
Silicone caulk
Metal bars
Screws
2*4 or other wood pieces
A length of seal (strip seal for doors)
High termerature black spray paint

Step 2: Choosing the Dimensions

I chose my dimensions taking into account the size of box I wanted to allow for enough cooking space. I used rimstar's super cool reflector size and angle calculator to determine how my reflectors would be set up once I'd decided the size my box would be:
http://rimstar.org/renewnrg/how_design_solar_cooker_sun_reflector.htm
I believe my box is cubic and roughly 35 cm and my reflectors 65 cm long.

Step 3: Making the Box

This step is simple, you make the box according to the dimensions you chose. I made mine out of 18 mm thick plywood and assembled it with screws and sealed the seams with the silicone caulk to allow as little air leaks as possible. Now, in the pictures you see that the wood planks were painted with high termerature black spray paint but you will later see that this was of no use after a modification was done inside of the box (see in the final steps)

Step 4: Making the Reflectors

I chose to make 8 reflectors and not 4 because when using only 4, part of your reflector is useless. They were all cut from the 8 mm thick plywood.
So 4 were rectangular: the width of the box and the lenght I had chosen (65 cm)
The 4 other were triangular: They are the same length than the rectangular but the width of the base had to be calculated
To calculate the width, the math come to the rescue:
We first calculate the dimensions of the dotted length (see diagram on 2nd picture!). Now that we have the angle (ß) at which our reflectors will be tilted (step 2) we can determine this dotted length using the cosine function:

Cos(ß) = dotted length / reflector length

In my case:

Cos(68°) = dotted length / 66
dotted length = Cos(68°) * 66 = 24.7 cm

Now, using the Pythagorean theorem inside the orthogonal projections of the reflectors (diagram on 3rd picture), we can determine the width of the triangular reflector's base:

(Base of triangle)² = 2*(dotted length)²
Base of triangle = 35 cm

(If this is not clear, go ahead and ask your question in the comment section)

Step 5: Making the Reflector Cone

Now that you have your 8 reflectors cut out, you need to assemble them into a cone. To do so, I cut 16 pieces of 2*4's at a 67.5° angle long of approximately 20 cm each. I screwed them onto the reflectors being careful to align the beveled surface of each piece. Once the 16 guide pieces of wood are srewed, you now have a way to easily connect the reflectors to each other: you screw the guide pieces together from the side!

Step 6: Adding the Mirrored Surface to the Reflectors

I used glass mirror that I cut myself with a diamond glass cutter. To attach the cut mirrors to the wood reflectors, I used little screws to be able to replace the mirrors if needed. Glass mirrors are heavy and a pain to work with so if ou can, go for stainless steel or acrylic ones!

Step 7: Praparing the Box for the Reflector Cone

Here, you need to bend metal bars at the correct angle to be able to attach the cone to the box. To attach the bars to the box, I screwed and siliconed them. The photos are probably expilcit enough for you to understand.
The bars were bent using my manly muscles!

Step 8: Making the Rotating Food Tray

This plate is very important because it allows you to keep the food level when tilting the oven towards the sun. It was cut from some aluminum sheet metal with a jigsaw, it was then painted black and screwed in place inside the box using washers to allow the tray to rotate.

Step 9: Making the Glass Door

First, glue the seal strip all around the top of the box. Cut the glass from the picture frame to the correct size, the glass I used was quite thin (2.5mm) but it works fine for me. I tried making some double glazing but it didn't turn out quite well... I'll try again some other time.
To secure the glass to the box I didn't use any hinges, I just used pins at all four corners.

Step 10: Mods and Tweaks

The inside of the box was initially black but I decide to test it mith stainless steel mirror instead of the black paint and for me, the mirrors gave way better results.
Using a progam called Crocodile Physics I saw why it was probably better with the mirrors inside: all the solar radiation is redirected towards the cooking pot (more simply, the food), heating it faster.

Step 11: It's Time for Solar Cooking Now!

With the mirrors inside, I manged to cook a brownie in 45 minutes in early april, in a normal oven it's 30 minutes so this oven a fast cooker!

If you have any questions, don't be afraid to ask!