Here's something interesting I just saw in a sales flyer:
Giant 31" x 40" Flexible Fresnel Lens - G13994
http://sales.goldmine-elec.com/prodinfo.asp?prodid=9039
DOesn't give the focal length though. I saw someone at the Seattle UW street fair
using something similar to make jewelery; melting various colored glass
rods with sunlight to make "raindrop" shaped earrings.
DON'T leave such a large lens indoors without covering it. If a sunbeam
should hit it wrong, even the reflection from the grooves can start a
fire.
Something that does work: solar-powered wood burner. Get a metal-jacketed
fiber optic bundle (glass fibers, not plastic, sometimes these show up in
surplus catalogs.) Stick one end at the focus, use the other end to
write your initials on a 2x4. But the end of the light-pipe at the focus
gets cookin. Probably needs a metal block with water cooling if you want
to do it right.
The same catalog also has 400nM UV LEDs for $0.99. For Burning Man, cover
your body with tattoos of tavern-rubberstamp invisible ink, then wear an
ultraviolet tiara? (Or buy those expensive UV-fluorescing contact lenses,
for Blinking Alien Eyes effect.)
http://sales.goldmine-elec.com/prodinfo.asp?prodid=7291