Found in Flan's Modern Warfare pack, Fiberglass is a valuable material. It has several uses, and is easy enough to craft.
Fiberglass is used in many crafting recipes for modern guns, attachments, and occasionally their ammo. It, along with iron, is a necessary material that is worth mass-producing.
Fiberglass is crafted by smelting a clay and sand mixture.
A block of sand and a ball of clay is all that's needed to create the mixture. Given that a single block of clay will produce four clay balls, it is easy to mass-produce Fiberglass if the player knows where to look. Rivers are a prime spot for clay, and one patch is often enough to give a stack (64) worth of it.
In either normal crafting menu, a block of sand and a ball of clay will produce the mixture. By the time the clay and sand mixture would have been created, the player should have access to a furnace. Smelting one mixture will produce a single product of fiberglass, but as mentioned earlier, you should have enough fiberglass to mass-produce it.