– Great progress Dave.
I know it is a bit late now, but the advice I got on here about fibreglassing joins was…
Carve out a ‘V’ shape between the two sections to be joined together.
Then build up the join with strips of fibreglass matting.
(Thinnest at the bottom of the V and thickest at the top.)
Also re-enforce the join from the underside (as no one will see that).
That way, when you sand back the ‘sunny side’ there is still strength in the join below.
Also, by the time you have finished with the filler and primer, no one will know.
I know it is messy working with fibreglass ‘upside down’…
But I’d suggest you add some matting from inside the boot opening onto the underside of your join.
Good luck, Paul.