One layer of fiberglass will not be enough on that size of a panel.
Adding multiple layers of resin will do nothing except make your fiberglass piece more brittle. You need to add layers of resin soaked fiberglass mat to add strength, preferably when the whole project is still "wet." If you do one layer, let it cure, then another layer, the two layers will not be "bonded" together very well. Alot of resins have a wax type substance that rises to the surface when it cures. You need to sand/scuff the fiberglass each time you let it cure if you are going to add another layer. And like i said before, always do something to keep your mold (in your case, your factory panel covered in saran wrap is the mold) straight, like with a 2x4.