My Weissman is fiberglass and I am pretty sure they all are but I have never had a close look at a BMW lid.For the hardtop there is no discussion if a fiberglass would be good enough (instead of metal) as the standard ones where fiberglass.
Also, there's the liner and fitting kit to consider. That would make it more expensive than a used HT In the UK but would likely be about 1/2 the cost of a used one in NA. And as you point out it would be new!! JIMI believe I could make a hardtop shell for about 350€ (just the fiberglass).
I don't know what the rear windscreen and rubbers would cost to have it finished.
The finished product could be more then the 500£ mentioned here....but then again mine would be a new hardtop, not a used one.
For the hardtop there is no discussion if a fiberglass would be good enough (instead of metal) as the standard ones where fiberglass.
Regards, Koen