Ok, so with a short delay, but here is the fibreglassing update.
I waxed the plug with a couple of layers of mould release wax, and after an hour or so starded the mess. I prepared lots of smaller sheets of fibreglass matting and polyester resin. Once resin mixed with hardener its kinda race with time, as it turns into gel pretty quickly, depends on ratio. I laid just like 3-4 layers of this matting as its thick. I've also got a nice fine woven sheet, but I will use that one for the actual part not for the mould...
And the best bit at the end. For some reason, I simply forgot to take photos of the whole process.
... but I've got the video: