Ah, well, I've seen a company advertised in Banzai which will make you up a custom bumper but I imagine it would be fairly pricey and I think they have a minimum order of 5.
Other than that I've never seen OE style bumpers in fibreglass.
And going on my own experience of fiberglass bumpers, I'd say they'd be the same if not heavier than OE bumpers.
Best bet would probably be find a second hand set of EK3/4 bumpers and use those for trackdays - would be cheap, the paint wouldn't have to matter and you can swap front & rear bumpers in about 10 minutes flat...