Closer to home, there's a workshop making these holders (and A-frames) in Cushendall, Antrim. They're just across the road from Redbay Boats and make their s-steel fittings. Redbay sell the polybottles (bit pricey).
.... And to save the lads in the Kingdom a post, I'm sure Gaelforce Ribs do exactly the same thing too, only better......