Fabricate a plug, keep it in the trunk.
I have run a 69 sixpack scoop, with the factory pan under the element. It has a lowered level, lower than the filter, with drains on the corners. Factory setup, works fine in the rain.
Also have a car with the dart raisin bran scoop, been run sealed to carb, and unsealed. (Strangely, unsealed was faster at the strip) . Ran in the rain, without an air cleaner at all. Also, not a problem.
I try to not run the car with no hood in the rain.