Howdy tarheel All the roofs I've seen have stiffners across them to keep the roof from wiggleing in the wind.
Take your hand and while pushing up slide it up and down the liner and see what is up there.
See if you can find a stiffner just where the sagging is.
When you find it measure the length of aluminum you need and get a piece from the depot or farmers supply or others..
A 3/4 X 1/8 X 48" should be available.. Then measure equal distances where you want a screw.. every 8-12 inches would suffice. Drill holes in the aluminum that the screws will slide thru without cutting threads.. Then with some help hold the aluminum in place and insert the thread cutting screws and sink em! Only problem would be screws too long would whistle in the wind..
Big JIm