I am installing a frame less sliding shower door. The width of the stall is a little over 60 inches which is a little more than the horizontal pieces of the frame. Thus, I need to place the vertical pieces on strips of "something" that is waterproof.
Question: Is there any such material available that is essentially waterproof on which I can attach my vertical frame bars ? I thought something like plastinated wood i.e. faux wood... is there any such material?

You can buy Veranda or other brands of PVC trim. I use it for bath and exterior work.