PVC is great for your application. Just place the straps every foot or two, and the sag will not be noticable. Runs of more than 6-7 feet may have problems with expansion that would pull the fittings apart. If you come and go at the ends with a 90 degree, expansion will be taken up by the sweep, thereby avoiding an expansion sleeve. Lightly sand, and paint the PVC and it will blend into the siding on the house.