I tried keels on the tail. I put a bit of toe-in to the keels and that acted like brakes, really slowing the board down. I could turn, but the board really wanted to go straight.
So I relocated them up to where they would be on my simms style board, without the toe-in. The braking effect was gone, but I had to be up near the nose to get a turn out of it.
Fine, so now I'm going to try a small single fin in the center. I'm hoping it will give my just a tad bit of stability in the tail. I think that with all that sharp rail, it does not need much of a fin at all. In fact, the addition of a fin was really to allow me to get an idea of how the bottom shape performs and then remove the fin.