In my Venice (Italy) lagoon with 8,5 I always use my Fanatic weed fin 34/48. Correctly Fanatic wrote both numbers, depht and lenght. It works brilliant and with 34 cm I can plane on very shallow waters. The rake (angulation) is more than 45 degrees and it has a wider chord and a smaller tip. It works brilliant with the old Mistral Echo 138lt that I still use in Venice for low winds. Slow and progressive stop when water level become very low.
On the opposite I also own a Prolimit 32/40 that has lower rake as you can see. With it board stop little more suddenly, also for the more traight tip shape, I suppose
I remember that in Select fin range there were two seaweed fin lines, one with a really huge rake for deep seaweed and/or very shallow waters, so this could be the solution for you.
Also Gsport-Curtis have a huge rake weed fin http://www.gsport.com/images/fins/big/weed_big.jpg
Imho don't think that is a solution to use smaller fins for heavyweight people like us with big sails
Have good wind