The focal assembly is simply four angle stock pieces set up so that they are screwed to the corners of the concentrator and meet at the center. If you use 8 foot pieces, they will meet at a point about 7 feet from the center of the mirror assembly. This gives you about 1 foot between where they meet and the focal point so you can mount things easily at just about the exact focal point.
NOTE! When you're not around the concentrator make VERY SURE that it is pointed in a direction that the sun will NEVER be. It WILL catch things on fire. Think very carefully about how the sun moves and remember that it traces out an arc in the southern hemisphere (northern if you're an Aussie).
The broom below took only 30 seconds to burst into flame. I cannot stress enough that concentrated sunlight is VERY dangerous. Also much fun if you're careful.