There was a guy who did this on here.

He placed a large aluminum plate on the underside of the hood that he sealed to the hood and the top of the aircleaner.

Rain won't be a problem if you give it a place to run off. T/A's AAR's etc had provisions for water run off.

Put drains in the rear corners and a lip around inside edge of the Air Cleaner seal. The lip must be high enough to prevent the water from draining into the Air Filter and force it to the drains.

I do not think they could flow that much air anyway because at speeds they would be under the boundary layer caused by the air forced over the grill.