You could just attach a tracker directly to the stmap it generates but even with a small target box size it may get confused by pixel values in your target box, distorting locally. What you need is a way to pick one pixel colour in the stmap and track it around the image as it moves. Magno Borgo has come up with a way to do this and has rolled it up into a simple nuke group you can use (note you can always swap out the second node, which auto tracks the max luma, for a tracker node if you prefer what it offers you, but I kind of like the way he does it. Here is the link to the nuke group and below is his video discussing it.