www.peterman.dk/windsurf-DIY-formula-to-slalom-fin.htm

Brilliant - so simple, been considering this for a while. I like the way he skips over 'getting the fin perpendicular' to the board. Not as simple as it sounds and so critical, hardest part I reckon.
Know what I will be doing this weekend.

Can you buy fin 'blanks' anywhere ?

- Like a head cut the right size and shape, but with the blade just a big thick square lump you can then shape yourself.

Would it not be pretty easy to shape a blade like this, just mark it out, cut it and sand it ?

Maybe pretty hard to get something good, but would be interesting to experiment

heaps of second hand fins around and it takes away alot of the sanding, just start with something bigger, close to the rake and foil width you want. Moving the wide point is a little more tedious, easy way is to take away some of the trailing edge- look for slightly wider foils or fins set back a little further in base for it to work out good. G10 is excellent for this as you can see the layers in cutting/sanding, if your sanding is not even you will see a wave in the layer and you can also measure the layers so the fin doesnt end up asymetric.

Quite a few people have made some good fins like this (and was the go before decent production speed fins hit the market). Alot of people will say it's too much effort and I guess it is but it doesnt take long at all once you get good at it- the trial and error adds another element to your sailing also. I've seen quite a few handmade fins which are cleaner than production fins, no flatspots or warping etc so any comments that it may not end up as accurate as a production fin arent necessarily true. I used to alter fins a fair bit, now I just use production fins.