Two VERY IMPORTANT caveats when building a parabolic reflector:
1) It focuses even when only partially constructed.
2) It focuses even when not pointed at the sun. (The focused light is a bit blurred and is off-center from the "focus" point of the parabola, but it's still there.)
A number of people building solar reflactors have blinded themselves, set their hair on fire, or otherwise received serious burns while constructing one by leaning into the focus as they reached in to install another component.
Remember that you can't see the focused light without dust or smoke in the air (which, of course, you don't want since it could damage your reflector.)
So use care. Like by building it after dark. (And by using small lamps rather than a 500 watt halogen spot, too.)
And be careful where it's pointed while under construction, so you don't set something on fire when the sun comes up, or tracks through a region of the sky where it puts a focus on your scaffold/workbench/tools, the side of your house, a tree, or the grass.
