I had the same problem with my Z. Though, after reading these posts, I wish I would have corrected the problem differently.
I sanded down the entire area, from inside the car. As, I had everything out from the Painless wire install. I fiber-glassed the entire area from the inside, all the way up the air horn. This mostly solved my problem, as now I only get a few drops in my passenger side floor.
As for rebuilding the air horn, I would either weld up a box, or make it out of fiber-glass. I would make it out of fiber-glass, as a welder is not easily accessible to me. Since I don't want to tear into this again, I am planning to seal off the right side of the vents in my cowl, and only leave the driver side vents open. Not the best fix, but will solve water coming in.