Yes Mcmaster carr is great.
I am thinking that rather than try to make a really super durable seal and protect the firewall and its cable ports, I might just make a fiberglass air splitter. This would keep any water off the cable entry points, as well as the cables themselves. This way I don't need to worry about having a super perfect seal. A continual blast of air and water would easily force water around grommets after time.
In the picture below, the curved portions below the rectangular box (the radiator) are the profile of the air splitter.
-Tony
