Functional cabin air intakes

Building in a Vintage Air a/c unit. I am looking at ducting the l/h nasa duct to feed fresh air to the cab, and ducting it to the a/c unit.

Strikes me I need to have plenum of some type to stop rain water etc from being ducted in as well. An open/close system would be an added advantage.

I would be interested in what other clever gents may have done.

While I've haven't actually done it yet I've looked into the problem.

I don't have a nice diagram to hand but after you have the inlet and have the airflow in a tube you want to expand the lower section of the tube and then have a vertical plate coming down from the top in the middle of that section. Then a drain in the bottom.

Any rain in the incoming air hits the plate and drops.

Essentially its kind of like an U bend, air can flow easily but the inertia of rain causes it to hit the side and then fall down to be disposed of.

I tried hard to find a diagram but failed, you can kind of see the shape needed from the outside of motorcycle ram-air ducts. eg