I have some experience with glass. And some with molding it around stuff.
I would think that if you were to do it, pull the plastic out of the metal frame, and mold around the outside. Easier to pop off. The sheets will lay flatter and can be squeegied out to remove bubbles. This is more awkward though for a first timer. But by overlapping the sheets like paper meche, you should get thicker, inter woven shell. I question though, if this is actually cheaper than IBC's. In DFW, TX fiberglass in this volume is expensive. 55 gallon barrels at $15 each are cheaper, and less risk of me messing up. Just cut in half length wise. But man are they ugly.