The contractor seems very unsure of procedure. He has put some very thin plastic around the front edge of the curve, holding it in place with wide black plastic tape. It is about 1.5 inches tall. The plastic strip looks like something that would be a corner bead for sheet-rock, not for concrete. It has holes in it. To close the holes, he taped it; and he only taped it after I asked him what would keep the concrete from oozing into the holes and binding that plastic to the concrete so it couldn't be removed easily when the concrete sets.
He says after he puts the concrete in that, he will pour it deeper and put another plastic strip upside down on the edge to give a rounded finish, and smooth it off with a trowel. We want a seat about 3 inches thick, and he says 2.5 inches is enough.
I asked how that concrete is going to stay there without a mold, or bender board, or something, while he is working on it with a trowel, etc. and he testily replied that it would be fine.
He plans to pour this in the morning. This question on this site may be too late for us, but may be of interest to others.