I just used a nut and machine screw, with a lock washer under the nut and "fender washers" against both plastic bits, to resecure our flopping air dam. I think I drilled the screw holes out a bit to take the small (3/16 or so) bolt. Once you secure the end, the crack will close up and be a small issue - maybe just overlay a scrap of ABS plastic from the inside with drain plumbing adhesive. Epoxy and superglue won't last.

A fender washer is a flat washer that is somewhat bigger in OD than the normal one for a given bolt size. It spreads the load when securing thin sheet metal or plastic parts, like, well, fenders and air dams. Normal hardware store stuff, and often easily scrounged in the home shop.

A machine screw is a bolt with a head meant to take a screwdriver instead of a wrench. Not as strong as a machine bolt, but convenient.