As I was thinking about the geometry of this joint, I realized there were at least three ways to adjust it so that it fits the way you want.
Method 1 - Chasing the Smudge
I have to thank Matthew Wajda of The North Bennett Street School for putting a name to this technique. I've used it in countless ways, not just for woodworking, but I've never known what to call it.
This is a lot like it sounds. You mark one edge of the joint with pencil, or in the case of the photo above, a lumber crayon. Knock the joint together, then back apart, and look for the smudges where the pencil or crayon have transferred to the other side of the joint. You can use this method with nearly any type of joinery, but if you plan to glue the joint together (dovetails, for instance) go easy on the pencil lead. And don't use crayon. This was just a test joint, and the final version will be a temporary joint, so I got away with over doing it for the benefit of the photos.
There's an important point to remember here. Lots of evenly dispersed smudge means your joint is clean and well fit. Look for the "bright spots" or areas that have more marking material transferred onto them. These are the places that need extra attention. If you get even smudge along the length of the joint and it still doesn't slide home then consider trying one of the methods below.
Method 2 - Plane the Sides of the Batten
You may have noticed in the first photo that the sides of the batten were still pretty rough from ripping them to shape. I should probably have cleaned them up before laying out the dado. But, since the initial fit was too tight, I decided this might be a good time to take care of them.
This method of adjustment can add up to big changes quickly. Don't go this route if your joint is almost perfect and you're looking for that last little bit of fit. By the time I was finished with the clean up, the fit was 85% there.
I should also mention that you need to be careful not to change the angle of the sides. Take a consistent amount of material from the entire face.
Methods 3 & 3-1/2 - Thin the Batten or Deepen the Dado
You can go either way with this one but, thinning the batten by planing it's wide face, is usually the easiest route to take. It's also the best way to sneak up on the final fit. The finer you set your smoother, the sneakier you can be.