I am wondering who people, like the Roman architects that designed the aqueducts and other engineering marvels, would pray to to make sure their designs were good and would work?
Vulcan was the Roman equivalent of the Greek Hephaestus (God of blacksmiths and fire) for engineering. And Minerva (Goddess of crafts and knowledge) was the Roman equivalent of Greek Athena for the science of architecture. Mercury was the Roman equivalent of Greek Hermes (God of travels) for transportation infrastructure? I guess. (Also in Greek culture we have many water body gods under Poseidon, similar expectation for the Roman Neptune. Further each large river, Lake or source of water has its own deity, so each aqueducts source of water would have a deity/ies.)
Probably Vulcan, god of fire and metalworking, among other things.
Minerva might have some influence over works of great intellect, but most Roman engineering tended to be done by rule of thumb, rather than calculation. The large safety margins this approach demanded are responsible for the survival of so much Roman construction to the present day.