The epoxy casting mixture should be resin, slow hardener (not very
slow) and high density filler to ketchup consistency. You have to layer the cast, pouring layers of 10 mm (3/8") when the previous layer is just starting to cool down. You do not need to add any fiberglass
with this mixture (it even better without because you can't wet it out good enough with the mixture, and get into other trouble when you wet it out before putting it in).
The reason of not using the very slow hardener is strength I think. The biggest problem is going to be the heat. Put ice around the mold
and start with thin layers, increasing each new layer with one pump resin/hardener until you think it gets hot enough. Too much and it starts boiling, which weakens the cast (yes, I had that once and had to start over ;-)
Make sure you have enough epoxy & filler because you want to finish it completely before cure.
All above is from memory, I'm pretty sure it's correct but can look it up if you have doubts. It is described in their book.
Don't forget to wax the mold
You could cast with threaded rods in place if you dare.
After cure, coat it with epoxy/hardener mix before sanding
(so pores are filled with epoxy, not with sanding
dust). After that, you can use a fairing mixture with microlight filler if needed. The last coating must always be resin+hardener so if you do any fairing, you must coat again. I would add some white pigment to last coat to help the painting (assuming it's gonna be white) and to provide some UV protection before the paint
After that, the hard part: painting it. Awlgrip 545 primer 2 layers and the "normal/standard" awlgrip finishing coat. Don't use the awlgrip 2000 etc. because it's weaker. I never like the painting part.