Q writes:

How do I keep the fibreglass from sticking to the mold?

Silicone grease spread thinly, or petrolatum (Vaseline), or Rain-X.

This isn't that difficult. You don't fancy mold-release products.

You can also craft your mold out of that cheap pink polyester Bondo.
Unlike the MDF, you can add it as well as take it away.