I love this thread.

My dad used to fix busted ATV fenders by drilling a bunch of tiny holes on each side of the crack and then "sewing" it up with some fine-ish nylon twine, then covering the seam with clear silicon.

I bet where visibility doesnt matter you could use something like PL polyurethane (In the caulking section at lowes/homedepot) over the seam or something else that would hold up much better than silicone.