I have looked and tried to find a reference to the thing I was describing, but I keep getting the wrong thing.What I am describing is a piece of aluminum railing about four feet long that you clamp to the plywood and run your saw along the edge, keeping a straight line. I suppose that you could do the same with a length of wood, but then you would not get to buy a neat new tool.
I have seen these in the woodworking magazines.