2 part epoxy for plastic, its usually in a black tube. that will work.
you need to cealn surface with a strong solvent, like denatured alcohol or mineral spirits, womens finger nail polish remover will work too.
ruff it up with some 220 grit sand paper and that will work fine 4 you.
the other way is with a a atv fender repair kit, its a small iron and comes with polyethene stick to fuse / melt / seal cracls in plastic.
ebay has them, but they not cheap.
so best bet is the 2 part expoxy for plastic.
Sporty