Activity with Cost But Duration is Dependant in P3

I have an activity, "Concrete Material Testing", with an associated cost which I want to mirror in terms of duration with "Concrete Paving". In other words, Material Testing should start and end with Concrete Paving. Concrete Paving’s Duration has changed a lot because of the constant design tweaks.

The first idea that came to mind is to use a Hammocks Activity, but I have always thought of Hammocks summative activities and not genuine activities.

Is there something wrong with this approach or is there another or better way? I am working on a very basic schedule, but there are constant revisions.

Thanks.
BEN