Hi Hugo,
Second what Jay says. 6 lays of 4oz should be plenty. If you use carbon fiber, the carbon is usually about 6 oz weight and would have the same thickness of about 1.5 layers of 4oz glass. If your curious, I make mine a little beefier. My commercial ones are a clear gel coat of UV epoxy, a show layer of carbon fiber, three layers of 9oz tightweave and a inner layer of carbon, all vacuum-bagged to squeeze out the extra resin.