White schedule 40 plumbing pipe from Home Depot; comes in 10 foot lengths. Every six or 8 feet or so, you need to add a flexible coupling to dampen the vibration from when the boat is running, so that the pipe doesn't crack. Home Depot also carries them, they're commonly called FERNCO couplings, used to make repairs to drain and vent lines in your home. Made of black rubber. They come with clamps to attach them, but change the clamps because the ones that are supplied with the couplings have galvanized screws on them, which will rust. Change them to clamps that are all-stainless steel, even the screw.