Help. Under carriage plastic flapping. How to seal?

I would go to a metal supply yard and pick up a strip of 1.5" wide x 1/8 or 3/16 thick steel that stretches across the frame. Then two self tapping screws at each end and into the frame and it will be up for good. If you're worried about corrosion you could drill and tap, or just drill and through bolt with a stainless steel bolt. It will hold better than any glue or tape that you're going to find easily available.