Here is a little selection. 
Why a Freestyle fin? Because they sail well even for straight cruising while being very short!  A 20cm FS fin easily takes a 5.5 sail (go on a broad reach for quick planning a little like a formula).  
The list below shows most WEED SPECIFIC FS FINS on the market. They would be terrific at rush Lake.
Makani fins - Naia
http://wind-nc.com/hatteras-windsurfing ... fins-naia/  (18cm for 4.7, 21cm for 6.0)
Chocofin weedstar
http://www.chocofins.com/where-to-buy/shop/weedstar/  (18 cm should be fine on a 5.5)
(US distributor: http://www.next-sports.com/ )
Kaku freestyle weed
http://www.avonsailhouse.com/kaku_fins.html (18cm for 5.0/5.5)
http://www.2-rad.com/Kaku_Freemove_Weed ... -fw00p.htm
http://www.mauiultrafins.eu/index.php?o ... 45&lang=en 
http://www.mauifin.com/fins/weed-line/weed-freestyle/
			
		