Huh, generally fiberglass will hold its shape well. heat from sun on the black and the way they are mounted tends to let it sag I suppose.
This will sound drastic but it will fix it for life.
Up in the country where you live are a great deal of machine shops. Some one local "hopefully" will have a line bore machine. The line bore machine can bore a very accurate hole length ways through the spoiler. you then can insert a 1/2" rod full length and epoxy it permanently in place. Repair the one end where you inserted the rod. I have not seen the center of of a spoiler. Hopefully its not foam filled? If so this approach probably wont work.
Another way would be to use a router or mill and route a channel on the bottom side then epoxy and mend the channel with fiberglass. Nobody would be the wiser on either repair.