How do you intend to attach the fairing to the rudder? With rivets or screws?
If you want to make it removeable you have to plan for that in the rudder horn area.
As for the rectangular area, I typically save pieces of the fairing fiberglass and after trimming to fit, just bond it onto the rudder and it matches the fairing. Just looks like a skin line between the two.
On the leading edge curved piece, I would cover the rudder pieces with packing tape or a release agent and use flox or cloth on the under side of the fairing, basically thickening the fairing at that point, and then sand it down to fit. You could also try to heat it up and bend it around but you would need a mechanical fastener to hold it in place since the cured FG has a memory and will eventually want to spring back out.
The gap from the bottom of the rudder skin to the top of the front piece of fairing, I would leave alone. You will never see that anyway.
All of the previous ideas are also good ones, depending on how you want to do it.
Good luck