Answer: You don’t. You let it in, and let it drain right out.
My buddy Jeff Miller (Millers Garage) is building a ’34 Cabriolet with a rumble seat, and sent me photos of his trunk/rumble floor when I posed the question to him. There’s a little “dam” on the trunk floor, and drain holes behind that in the floor. Water runs down the rumble lid, into the trunk, and just drains out the holes.
I had some oak 2×2, lots of resin and mat, and made the dam out of oak and encapsulated it with fiberglass. There was a hole in the floor for the previous frames Model A crossmember which I glassed up, the dam is over that. It does double duty as a stop for the rumble lid.
It’s not pretty yet, but a little skim coat and some bed liner will make it look OK.
In other news, I got a couple of screen door turnbuckles, and put on in the drivers door, which had a persistent bow. It stuck out at the bottom about half an inch from the door jamb. The turnbuckle mounts at the rear, top close the the hinge and at the bottom front to a piece of plywood inside the (untracked) door skin. A couple turns of the turnbuckle drew the lower corner in perfectly. I’ll put one in the passenger door tomorrow.
I also discovered, to my surprise, that the “B” pillar or rear door post is steel, as is the back inside framing of the doors, where the hinges mount. The front door post/cowl has no reinforcement at all, it’s just an “L” lip of glass with no framing, either wood or steel, behind it. I’m going to glass a strip of oak behind that tomorrow, which should help brace the cowl. It has a lot of “flex” right now, owing I’m sure to the fact that there’s no bracing in the body there at all.