usualy the reason the smell doesnt go away, is because you have some uncured resin in the enclosure still, i suggest taking the enclosure out. look it over and find the areas that are still some what uncured sticky or soft. one way you can get that area to cure is applying an extra layer of fiber glass resin that has enough hardner in it to fully cure it self and the uncured resin that youre trying to take care of.
if you cant find the uncured area, apply the new layer of resin on the whole enclosure it self. that will create a seal around the whole enclosure, let the enclosure cure for at least 4hours or over night before you install it in the vehicle.
another way to get the smell away is to take the enclosure out of the car, take the sub out and any plastic hardware. then build kind of an oven around the fiberglass enclosure, lay a couple of wood panels around the enclosure to keep the heat in, and stick a heat gun on one of the oven oppenings. get the enclosure as hot as the heat gun can get it for a couple of hours then let it fully cool down. that should accelerate the curing prosses of any uncured resin in the enclosure.
none of the fiberglass enclosures i have built, have keeped the resin smell after the resin has fully cured.
good luck with it. let us know if it worked.