On Swaylocks I came across this board. Zillions of options to mount the fins.
The 4 way finsystem is interesting. On their website you can read what is possible
Talking about the Swaylocks forum.. absolute my favorite site where I collect the most of the info I need. Another forum where I collect loads of info, and what I like to read is this one. The homebuild sup boards come close to windsurfboards.