How does this sound as an alternative to molding the channels from latex tubing? Drill the holes in the deck, bend a section of brass tubing into a "U" shape and insert into the holes from below. Then flare the ends of the tube with a flaring tool (for fitting gas lines, available at hardware stores). Seal the tubing/ wood fitting with epoxy or silicone. Underneath, wedge a "U" shaped piece of wood into the loop of the tubing to stabilize it and fibreglass over all beneath to provide stability and to waterproof.