Could is a pretty loose word. You could make a rack out of hamburger meat, but wouldn't it start to stink?

I don't see why you wouldn't be able to make a rack out of fiberglass, I'm just not sure why you would want to. It would, I imagine, be VERY expensive, since you would need thick sheets of fiberglass. Much more cost effective to use some of the more conventional materials, and I would think that xpvc would be more durable and stronger anyways.

Unless the idea was to make a see-through rack, which could be cool, although it wouldn't do much for making the animals feel safe haha.