Yes, the mirror tile is glued to a 3" square 1/8" thick steel plate. The 4 arms of the spider are also welded to the same steel plate and a 1/4-20 steel bolt pushes on each corner of the mirror. It takes very little pressure to distort the mirror tile, i.e. only a couple of turns on the bolt, since the focal point is far away. Each mirror is adjusted separately while focused on the front of the oven. The oven is also on tracks, so it can move back and forth, because the “color” of the light changes the focal point throughout the day.
Glass at the focal point was a problem for me. I tried strips of glass made from the glass in a copy machine, but they all shattered. Next, I tried Pyrex, and even changed the mount somewhat to hold the rotating tray from a Microwave oven. Every glass I attempted to use eventually failed, so my current solution is a heavy steel plate mounted at an angle inside the oven, and backed up with another 100 pounds of steel mass. The steel radiates heat into the oven. This idea came from India where a Scheffler Reflector was heating hunks of ships propeller shafts heated by the sun and rotated into a kitchen for cooking.
You are correct in the danger of this setup burning things. One builder left his unattended and the beam melted a plastic bumper cover on his car. Grass can also be ignited. In the photos, I have the array facing the opposite direction, and it is stored in the barn when not in use.