Here's your solution,save hundreds on elbow grease and end up with a better fin.
Just do a couple of hours overtime at work goggling makani instead of sanding or reboxing. Easy peasy and any size you want.
I had it done re-boxed, it works, especially with G10 fins where the performance comes from the foil design and not layup of carbon fibers ... it will not be stiffer if cut from base, not even closely as those fins that are re-sized by sanding the tip as the chord will usually be wider and the whole thing thicker, among other things ... DunkO's drawing is exactly making the point!!
If you do not believe, try it ... it is much easier to make a new box than mess with shaping the fin by sanding the foil by hand ... you must have some guys who do this kind of repairs, do not tell me you don't!
This is an example of both the foil tip reprofiled and reboxing. Reboxing is for a weedie of course, but using a table topped belt sander and finishing off with the mouse sander, then wet sanding with 240 and 400 wet and dry, the fin tip can be done in a matter of an hour or so. Reboxing will take substantially longer. If I'm doing the tip of the fin, I'll make sure I work at least the top 6" (fin tip), making sure I keep to the original chord ratio.
Btw, this is only to show my side of the discussion, and appreciate both sides for and against. 
It certainly is Bender. 31cms @ 43 deg with a slight drop off near the tip for those annoying ground attached weeds where you are.
1. NOBODY is as accurate at hand foiling a fin as a CNC machine.
2. Tiny differences in foil shape can make a huge difference to performance.
OTOH
1. Even theoretically bad foil shapes can sometimes do what you want from them.
2. Sometimes, close enough is good enough, for some people.
Having said that; that is a great job Pepe. 
That is also one very nice base mould!
Cheers Sailquick, and I couldn't agree more. Just one thing I might add is, I would expect all high performance fins to be cnc'd, also if they've been marketed as such. However,.. this is why I jokingly refer to my belt sander as my "Thai cnc machine"
