Hardcore Rough Water Slalom

[surfrider]

I want tips for smaller slalom fins for rough water.

Last weekend I sailed 1 day of the Belgium Championship.
We had a lot of wind, chop up to 70cm.
Most started with 6.0 - 6.5, I started with a North RAM 6.3 on my S-Type 104 2007 (my high wind rough condition slalom board) with my C3 D2 36cm.
Total control, speed very good. Made it to the first 2 finals. Just before the second final wind picked up even more, almost the entire field was early so we had a restart.
My 6.3 was now too big. Switching to a smaller sail was not really an option. I don't have any controle slalom fins for my 5.8 and 5.4 Warp yet..

A smaller size C3 D2 would be great, but cannot find any. F4 D2 (same fin) is a bit expensive for the limit amount of usage. What would a a good alternative and in what size?
Select Devil, Deboichet SL3 or SL5? MFC SL (not the SL2)? Tectonics Falcon F1/Goldwing?
What would work under the S-Type for really slalom competition?