Frankly, I am a bit unsure what your photos are showing. The bottom one, I don't even have an idea of the scale. The top one, my best guess is that we are looking upside down. It seems to show a fiberglass collar (or partial collar) for the wooden mast support which seems to have rotted very thoroughly. I assume it would have extended in what is the "up" direction of the photo as presented and presumably rested on something ("above" of what we can see). Is that correct? (Is that "something" the one shown in the second photo?). What is the distance between the two? Or how long was that post?
Fiberglass work is in some sense dead simple. You get your resin and glass. You mix the resin per instruction, cut the glass into manageable shapes, put in place in layers and make sure each layer is wetted out. The skill comes in working neatly with this somewhat messy material including no using more resin than needed to keep the ration of glass to resin high.
I prefer to use epoxy resins for my repairs, and for consistency, always use the same supplier (SystemThree in my case). They publish an online publication the call "Epoxy Book" that is full of useful instructions. The key for working with epoxy is getting the ratio between resin and hardener (2:1 by volume for the stuff I use) very accurate, and mixing it thoroughly. I keep around the mixing container to check afterwards that everything cured and that no uncured epoxy is still sticking to the walls of it.
Is this what you were thinking?
- remove remaining support
- sand around it
- cut new wooden support to required length
- coat it in epoxy resin on all 6 sides and let cure
- place
- place strips of fiberglass to form a new collar
- wet each layer out with epoxy resin
- if necessary, wrap in plastic foil to hold together until cured
Seems workable to me (I added the bit about coating the post, because that's the only way you'll keep it from rotting).
If there's any complication that I should have noticed from the pictures, my apologies.