If you are after cross onshore winds on the lake this spot across from Milang on Point Sturt might be the spot if you don't mind going through the reeds. It looked pretty flat to day and the wind was pretty consistent. I imagine if you worked your way up wind it would get even flatter.
The track down to the parking spot has got a bit overgrown and muddy a bit like Boggy II when we first started sailing there.