Epoxy + microballoon = filler, light but little strength
Epoxy + chopped cotton or epoxy + colloidal silica (Aerocil) = Bonding paste (flox)
I believe West 403 = chopped cotton, West 406 = Aerocil, West 407 = microballoon, West 410 = really creamy, light colored, microballoon. There are other West fillers!
All can be mixed to what ever consistency is required for the job in hand.
A chopped cotton & aerocil mix can be used. Aerocil gives a smoother mix, chopped cotton produces a stronger mix. Bonding paste is tough to sand.
Peelply is used over a layup to make the surface look better and to make the next layer stick well (otherwise must be well roughed up).
In this case it will take 4 resin processes (4 days) to finish the fairing (flowing Van's steps, although 3 & 4 could be reversed). 1/16" ply could also be used instead of making a repair patch. Repair patch (or ply) could also be stuck on with super glue or 5 minute epoxy, that would mean only 2 resin processes. I would rough up inside of repair patch and inside & outside of fairing before starting.
1) Layup the repair patch, cover with peelply, allow to cure
2) Sand away corner and glue on repair patch, allow to cure
3) Sand back edge of repair patch to fair in, rough up outside of fairing well with 60 grit, and lightly sand outside of repair patch (don't clean with alcohol, just brush and vacuum). Apply layer over the whole lot (too difficult to apply a ring), a layer of peel ply will help achieve a better finish. Allow to cure.
4) Rough up very well with 60 grit - no shiny surface where epoxy will be applied. Apply bonding paste to inside edge (glass cloth won't stick to a sharp corner) and lay up the internal layer. Allow to cure.
Outside will have to be filled before painting.