Jig & Plumb

This will require two to three people. Find the exact center of the circle-slab. Place the finished jig so that the exact edge (centered) of the vertical base of the "L" is right on the center dot of the circle. The horizontal leg of the jig should be the length from the center of the circle to the vertical rebar, or (to be precise) the distance of the perimeter minus 3", minus the diameter of the rebar.

With a little help from your friends, you will hold the jig plumb/vertical at the center of the circle, with the horizontal jig-leg butting against a vertical rebar. Bend the rebar over and along the arc-channel toward the center of the circle until the end is overlapping the center point. Then, move the jig to the exact opposite rebar, position it as before and bend it toward the center of the circle until the end is overlapping the center point and the other rebar. Wire the two rebars together where they overlap.

Continue clockwise around the circle, bending each rebar, then its opposite, in turn; then wiring, until you have done all the rebar. As you join the bars in the center, you will notice some congestion. Simply overlap and underlap, wiring them as you go. It does not have to be perfect. For now, leave the two extra rebars on either side of the door alone. When you finish this stage, you will have the shape of a dome skeleton! Ideally, you can weld the connections at the top for extra strength.