Replying to Building rail

Im building my first rail, it is a double kink. I have already built the frame, it is about 16.5' long. I was wondering what to use for the top sheet, i have heard PVC, metal, etc.. Metal is too hard to work with. And as far as i know i can only get pvc in 10 foot lengths, so i would have to join pieces somehow to cover the the whole top of the rail which is about 18'. Anyone who has built a rail have any ideas or tips to help us out with what to use, how to attach pieces, where to get supplies, etc... Any help is appreciated. Thanks