Do you need a weed fin simply for "weed shedding" qualities,or do you also have a water
depth (shallow sand bars etc) issue?
I've had most of the Formula boards since they were first invented, and here's what has worked for me.
My #1 choice is the Lessacher Design 40 cm Formula Duo:
My #2 choice is a toss up between the Tangent Reaper Weed 46 cm:
the True Ames Shallow Water Weed 20"/51 cm:
If you have a little more water depth, the True Ames Santa Barbara Weed
works well on Formula boards in the 23.0 inch (58 cm) length.
You will never get the same feeling or upwind VMG with weed fins that you get with
a good vertical race formula fin, but if you move your mast foot back to near the back of the track, you can get the early planning and pretty good upwind performance that formula boards are known for.
Also, some of your "spin out" issues may be due the the extremely small sail size (7.5 m2) that you are using.
Formula boards (and virtually all boards over 90 cm wide, need large powerful sails, and you need to make some adjustments if you are using smaller sails.
Mast foot back all the way (very contrary to normal formula suggestions) to get the CE of your small sail back where it will balance "over" the fin better.
Hope this helps,