Your right. You don't want to skin to hit the frames. Otherwise you have a lump in your skin. If it is under water it's going to mess up the smooth flow of water. I use 5/8 and sometimes 3/4 stock but I move the corners further back, like 1/2" or so to make sure they never contact the frame. I don't think the dimension is critical, just as long as it is smaller and there is support for the stringer.