They are surprisingly not as expensive as I thought, provided you're ok to put some work into it. You can get one that isn't waterproof for under 50 bucks, and a fibreglass kit is about twenty, and waterproof it yourself... as opposed to at least 150 for a ready-made set up.

Although I will say, working with fibreglass is a miserable messy sticky smelly business and will take at least three or four days if you do more than a single layer. (apply layer, wait to dry, sand, apply another layer, repeat.)