Not sure if this is useful, but we built a concrete board weir and gate to control input to our pond. We weren't concerned about leaking of the dam and gate so just used boards. These are intentionally leaky but left open the possibility of adding gaskets. As you mentioned, Agri-Drain sells a range of items including stop logs, we are low tech. I included a few pictures of the weir/gate in operation, the structure dry, and the rebar used. The forms were all scrap plywood and dimensional lumber coated with soybean oil and sprayed with PAM. These can bet seen piled up in some pictures. We poured the pad off site then assembled the forms and lateral rebar on site and poured the walls. Worked fairly well.
Don't know if this was of any use. Your structure will be very different
