I don't know what the cover is made of, but some materials bond (glue) better by solvent cementing.
Materials like Polycarbonate, ABS and Acrylic will bond by just painting over the crack with methlyene chloride. it wicks into the crach, melts/softens the plastic that sticks to itself, then the solvent evapourates.
A crack indicates stress, if you have room, I'd cut a new piece to laminate over the cracked area to reinforce it. If using solvent , apply to both surfaces then clamp tightly together, and leave over night.
Hope this helps
Mike D