Just noticed this old thread and have a few things to add.
Your typical solar tracker isn't going to be sensitive enough to aim a lens with enough precision.
For cooking the best thing is a concentrating mirror, not a lens.
The lens will shine on the top of the pot, the mirror on the bottom.
Picture a satellite dish, especially one with an offset focal point. Glue some Mylar or tinfoil on the dish surface. Hang your pot at the same place as the signal pickup and point at the sun. A bit off from the focal point will spread the heat out a bit, not concentrated into a point.
It won't be so sensitive to position and the heat is where you need it.
I've always wanted to try it but haven't found a dish for free nor the time.
Careful where you put your fingers. Maybe sunglasses too.... sunblock....
