I haven't fitted any fins, skegs or rudders on my kayaks, except for the racing kayaks, but I believe that the fin needs to be a bit further back, probably at least halfway back between the cockpit and the stern.

I believe that a fin right at the back of the cockpit might stop sideways drift if you were sailing, but what you need to do is move the CLA back a bit. Imagine fitting feathers on the back of an arrow to make it go straight. You wouldn't put them halfway up the shaft.

Some friends of mine paddle sea kayaks with a lot of rocker. Their paddling technique is a bit ordinary, so the kayak doesn't track particularly well with the rudder up. Rather than have the rudder down all the time, they fitted a skeg right at the back of the kayak. It is a simple removable skeg held on with cable ties. It looks better than it sounds and gives them enough tracking to suit their very relaxed paddling style.

The shape of the back of their kayaks makes it a very neat, secure arrangement. It wouldn't be possible to have quite such a simple, secure arrangement on the tapered stern of your kayak. However a bit of experimenting with a small skeg held on with duct tape mighthelp you discover the best location to suit your kayak and you technique.

Someone with some experience of fins or skegs might be able to give you more specific information about the best place for a skeg.