hmmm... I have a different aproach... Take your calliper and confirm your "closing up" theory. Check it the opening on the castings is paralle. lIIRC the gap for the blade is about 25mm wide. Most of the Lexan blades are way thinner... mine are 22mm. The blades made in composite construction might be thicker.
Most of the lexan blades are not really flat/plane in the area where they fit in the rudder castings. Fourtunately the lexan blades will allow some sanding to get them back in shape.
Bending the castings might be possible. Remove all the plastic stuff on them. Warm them befor you start bending. Place them in a big vice so that your bending is more controled. Any Yes they are not cheap.

Patrick